Session Zero Guidelines

Critical Things to Cover in Session Zero

Scheduling Time to Play

Nothing kills games more than scheduling conflicts. Nothing.
Whether you meet face-to-face or play over video or voice chat, you need to meet regularly for a game to live.
I have managed to cryogenically freeze a game and then unfreeze it after a long hiatus before but this doesn't always work. In my observation, one should make a goal to play a session once a month at least. Less than this, and players will likely lose interest or lose track of what is going on. This can be alleviated with fastidious notes (I like World Anvil!) but long delays between sessions should not be routine.
During Session Zero everyone should clearly communicate when they prefer to play, when they can play, and what the odds are of an unexpected cancellation. There needs to be method of communication set up to let the other players know in advance in someone is going to be late or a no-show. It is understandable that stuff comes up, especially for adult players with careers and families.
There needs to be a contingency plan for what will happen if the group convenes one player short. Sometimes you can come up with a plausible in-story reason to have a character sit out. Other times, someone else has to play another person's character or the The Purple Cow must step in.
In Session Zero you should establish what the contingency plan for a missing person is. If you aren't using the Purple Cow, you have to make sure the rest of your group has access to your character stats and it is agreed who will play your PC by proxy.
In my experience and the experience of many other gamers on the internet is that it is usually better to set up a specific day of the week to meet every time (though not necessarily every week). If the game's meeting day shifts around a lot, that tends to cause problems.

Where to Play

Along with scheduling time, you need to schedule a premises, or if like me, you picked up video/voice chat gaming during the pandemic, you need to make sure everyone is on board with the same technology and you need a back up communication method (likely text messages) if there a glitch and you need to communicate ("brb eta 15 minutes") or something similar.
And if someone is hosting don't trash their place and help tidy up after a session.

What is the basic premise of the campaign?

The Game Master should have a basic idea of how he wants to run a campaign and should clearly communicate what his basic plan is so players can calibrate their expectations appropriately.
Players can and should provide input and a game master can adapt their concept based on what their players want but in my experience, a Game Master should never completely throw out his concept and do something he doesn't want to do because it doesn't end well.
If your planned campaign is based on deep dives into Game of Thrones-esque politics you probably don't want a half-orc barbarian PC who's only skill set is hitting things and causing them to fall down. If you are planning a pulp action where characters wander around the frontier beating up villains of the week, you don't want an aristocrat diplomat PC with no combat skills.
Likewise, if you planning on running a sea-based campaign, you probably don't want a player making a Mereshnari master desert survivalist.
All that said, an experienced player might enjoy playing a fish out of water type PC and that can be a lot of fun (though it can also fall flat). I am a big fan of the RPG philosophy that you learn the rules so you know how to break them.

How is long is the campaign is expected to last?

hour glass image on Pixabay by James DeMers

Sometimes a campaign is planned to be indefinite, "We will keep going until we get sick of it." Other times there is a clear goal. "The campaign is going to last until the main villain is defeated." A campaign can also span a major event. "The campaign is going to start at the beginning of the Rutabaga War and end with the end of the Rutabega War."

This doesn't have to be set in stone, but having a rough estimate of how long the game will last will give players an idea how much depth and background their character needs. The shorter a campaign is likely to be, the less a character needs an in-depth background. It also helps players calibrate their expectations.

What do the PCs want and why are they working together?

Sometimes it is entirely up the Game Master how and why the PCs are working together. For instance if the game starts with all the player characters in the same prison block awaiting execution, they will have to work together to escape regardless. Same thing if the PCs wake up lost on a desert island or extreme circumstances force the PCs to work together or die.
Those games can be fun, but usually campaigns start in a less extreme way. It is a good idea that each PC have a preexisting relationship with at least one other PC and a preexisting relationship with an NPC in the setting. This requires that the players collaborate with the other players and the Game Master.
If the Game Master is going to try something more heavy handed to logically explain why characters who never met will now work closely together, the GM should probably communicate this. It could be as simple, "you are all part of the same army and your commanding officer have hand picked you all from different units to form a new elite squad", but the GM still has to communicate this and the players still need to make character concepts that work within an army in this example.

How much Plot Armor is to be expected?

In other words, how easy is it for PCs to die?
No one likes to lose. If all the PCs die, the game almost always ends. Also, if a single character is the center of some subplots, those plots probably die when that character dies and a lot of the work the player and GM put it into that character seems wasted.
On the other hand, without the possibility of defeat, victory is hollow. The more likely a character is to die, the sweeter that character's survival and victory will taste.
To a lesser extant, how a character dies matters too and not just that a character dies manners. As a GM, I generally don't like to have PCs (or even certain beloved NPCs) die and often bend the rules, fudge the rolls, or retcon events to change things, but I am far less likely to fudge things if a character dies a glorious death battling a worthy foe than if some no-name mook gets a lucky shot in.
Then again, some players relish the challenge of a world where even a low level mook can bring death.
The players and the GM should communicate with each other how much tolerance for PC death/PC plot armor they have.

Useful Things to Cover in Session Zero

What is the tone of the game?

I think tone is a very important topic, but the reason I didn't include it under "Critical Things to Cover in Session Zero" because with some groups it might be better to discuss this in a series of one-on-one communications rather than one big group. Other times a Game Master has to logically deduce or infer tonal boundaries because people aren't forthcoming when asked.
Tone is nebulous, and an RPG can (and probably should) have variances in tone from session to session. Tone is many things.

Comedic versus Dramatic Focus

In my opinion, this is the most important aspect of tone to consider. It's not a binary choice, a campaign can and probably should, have a mix of comedic and dramatic elements, but you need to make the sure Game Master and the players are roughly on the same page and the players are roughly on the same page with each other.
It is not a problem if characters differ in tone from each other in tone a little bit, but if all the players but one are dramatically focused and one is extremely comedic focused, this will cause friction that will only get worse over time. Same thing if most of the group is leaning towards comedy and one player is a focused dramatic role player.

Violence and Gore

Grittiness ranges from the bloodless action of a G-rated action cartoon to a Tarantino movie. Different players (and the GM is a player) have different tolerance and preference for different levels of graphic violence.

Nuance and Consequences

Nuance is tricky. If your characters are Wild West heroes that travel around toppling the Villain of the Week, do the players care if overthrowing the local despot creates a power vacuum in the local government or creates an appearance of weakness that will entice an neighboring nation into invading? Then the characters should probably routinely have to deal with the downstream effects of their actions. If the players are annoyed or bored by these things, "don't think about it Morty."
Scarterra as a setting lends itself well to downstream effects but it is not required that the player characters always have to deal with it.
Also along nuances, you can use an RPG as a vehicle to explore complex issues but in my experiences most players want to use games for escapism, not serious exploration of difficult topics.

Sex

GM "You're not there, you're getting drunk!"
Player "Okay but if there are any girls there I want to DO them."
-Reenactment of Dungeons and Dragons

Most players and most Scarterrans are aware that they were not delivered to their parents by a magic stork. There are a lot of RPG stories about lustful bards or sexy tieflings. Scarterra has satyrs which are defined in part by their sensuality, but satyrs are hardly the only Scarterrans that like to get drunk and have sex.
Some people find these sorts of things endlessly entertaining and some find them unbearable annoying. Most players find them amusing up to a point, The players (and the Game Master is a player) should communicate how much they are amused or annoyed by the sexual antics or drunken mischief.
Beyond the level of inclusion, there is the darkness of how sexual themes are included. It's one thing to have characters talk about wanting to get laid and it's another thing to go into graphic detail. One needs to be aware of the maturity level of players and their appetite for stories to include serious things like rape. Other serious issues include arranged marriages and bastard children among other things. In the real world, mixed race marriages and intercourse is usually not very controversial but in Scarterra certain types of relationships are controversial in some circumstances, see So you want to play a half-breed? for details.
It wouldn't be the current year without mentioning non-binary sexual preferences. Scarterra has non-binary people in it, they are often nicknamed "leafers". Some people find the term offensive, some don't. Scarterra has people who support leafer rights and inclusions and Scarterra has people who want to marginalize or discriminate against them. Scarterra also has people who don't care and they have people who do care a little but find it easier to pretend the topic doesn't exist.
Some players want to explore these sorts of topics. Some are annoyed or offended by their inclusion. A lot of times, it depends on how it is included. In my personal experience, when this comes up it feels like the player bringing it up is trolling. If a player is asking to play a cross dressing non-binary invoker while giggling I am not likely to believe that player was creating such a character to explore the complex roleplaying depths.

Food

Player 1: Where are the Cheetos?
GM: They are right next to you?
Player 2: I cast a spell!
Player 1: Where's the Mountain Dew?
GM: In the fridge, duh
Player 2: I want to cast a spell!
Player 1: --Can I have a Mountain Deeeewww!
GM: Yes, you can have a Mountain Dew, just go get it...
-Reenactment of Dungeons and Dragons

If you are meeting face-to-face, it's good to have a tentative plan at least before your start playing. In the past, my gaming group has wasted A LOT of time arguing about where to get carry out from.
Other parties have been torn apart by arguments about one or more players not contributing their fair share to group snacks.
Especially as players get older, you should make some attempt to have some healthy options, or at least not rely exclusively on Doritos and candy.
Also, if you have an older group, make sure everyone is on the same page for whether alcohol is acceptable at the gaming table and in what quantity.

Player Vs. Player

It's a good idea to figure out how the various players feel about player versus player conflict (and the GM counts as a player for this purpose).
Apart from a brutal political one-shot campaign, I would should suggest PCs never try to kill other PCs but with experienced role players, we have some interesting sessions as PCs conflict over non life-or-death goals or argue about differing methods.

Party Balance

Scarterra D10, and every d10 based game I am aware of does not have character classes. You don't have to worry about party class balance like you would in a D&D or Pathfinder game.
I suppose most players for Scarterra D10 likely have a background in D&D or Pathfinder, so players are naturally likely to gravitate towards making characters with different niches. This is a good thing, but a GM shouldn't try to force this in Session Zero.
Thanks to potions being relatively plentiful, a PC adventuring party doesn't NEED a divine spell caster or an arcane caster (though it is handy).
Because of the way the magic system is set up, you can also have an entire adventuring party completely made up of theurgists or an entire adventuring part made up of mages and have no overlap of niches.
When it comes to fighters in Scarterra D10, redundancy is good. Playtesting shows it pretty good to have the whole party be pretty good at melee because double-teaming an opponent to wear down their parry successes is a solid strategy.
A nod towards party diversity of talents is not a bad idea, but the GM (and the other players) should not twist anyone's arm to play a certain thing or not play a certain thing.

Character Story Balance

Sometimes one character evolves into the role or is outright named the party leader. Sometimes the broad narrative can focus largely on a single player character for short or even long periods of time
If one character is going to become a major focus on the campaign, the GM should make sure all the players are okay with this.

Dice

If you got a new player, either new to RPGs as a whole or new to the d10 system, are you going to make them buy dice or is one of your players OCD enough about buying lots of dice that they can loan them some dice?

Two Fairly Obvious Things

1) If you have been playing with a group for a long time, a light and informal session zero is probably called for. If you are playing with people you just meant or casual acquaintances you are about to play with for the first time, a more formal session zero is called for.

2) It is never too late to have a Session Zero. You can and often should discuss these things periodically for ongoing campaigns.