Identifying the Sub-Goals

Once the map is drawn, mentally recap the ultimate objective of the adventure and try to imagine how play might progress. What routes might the players take? What might they see or encounter along the way? Often, the easiest way to accomplish this is to break the objective down into a series of "sub-objectives" the players are likely to see as prerequisites to the main objective. In my own adventure, for example, the players' primary objective is to escape from their orc captors. They face an obvious obstacle in the orcs themselves -- it will certainly prove difficult for the unarmed and weakened PCs to fight their way out of the complex. Before they attempt escape, therefore, it's likely that the players will try to tackle one or more lesser objectives. They might try to find makeshift weapons or equipment to improve their odds in combat, for instance. Or they might search for spellbooks and material components necessary to restore their magic-users' spellcasting capability.
A careful look at the maps for "The Scar" and trying to "think like a player" inspired this list of sub-goals:

1. To gain weapons.

Since the unarmed PCs stand almost no chance against the numerically superior orcs, this is a necessary first step toward any plan that involves the PCs' fighting their way out of the complex and a prudent precautionary step toward executing just about any other plan. There are three obvious ways to achieve this goal. The PCs can capture weapons from the orcs, they can find their own weapons stored somewhere within the complex, or they construct makeshift weapons from the stones, planks, and other items they find in the temple ruins.

2. To restore spellcasting ability.

As the adventure opens, the spellcasters in the party obviously lack the material components, spellbooks, and other items necessary for making magic. Finding a method of restoring this ability would obviously increase their chances of escaping -- a single sleep spell might be all the PCs need to elude their captors.

3. Identify less obvious exits.

The temple map depicts only one obvious exit to the outside, and it is certainly well guarded at all times. To increase their chances of escape, the PCs might search for a less obvious exit that isn't so well guarded.

4. Identify a "sneaky" method of taking out the orcs.

It's clear that the odds favor the orcs in a straight-up fight, but clever players might look for a more devious method of removing a large number of orcs from the picture. Perhaps there's some way to trap the bulk of the orc tribe within a conveniently collapsed cavern or behind a locked door.

5. Identify a potential distraction.

If the PCs decide to leave via the main exit, they might improve their odds with a distraction aimed at occupying their captors. Perhaps they'll free and arm the remaining prisoners, or devise a method to provoke a fight between two rival factions within the orc tribe.

Enabling the Sub-Goals

With such a list in place, you can make sure that you address each of these possible approaches as you design the adventure. Of course, nothing says that you must design the adventure so that every single one of the potential approaches works -- maybe there aren't any less obvious exits -- but it's a good idea to enable as many as you can. That way, you give the players plenty of options. For "The Scar," let's allow every approach we can devise, making some a bit more difficult to accomplish than others. Since this is the first adventure in a whole new campaign, there's no way to anticipate how the players will respond to the obstacles we place before them. Consequently, let's give them as many options as possible to decrease the chance that they'll become frustrated while the DM is learning to understand their playing styles.

Think Like a Player

The list of possible sub-goals suggests that, to pursue any of these goals, the PCs must first resolve to explore the temple itself. Because it's impossible for them to form an interesting escape plan without gathering plenty of information about their surroundings -- and because their opportunities to gather such information are limited -- we should probably provide them with some assistance. One good solution would be to include a series of elaborate friezes depicting the construction of the complex, running along the temple walls. By learning to interpret the friezes, perhaps the PCs can glean useful information about various areas in the complex without actually visiting those areas. These clues can vastly augment their "intelligence gathering" capabilities.
Of course, it's extremely unlikely that a list of potential objectives covers every possible approach the players might pursue. In your games, you'll quickly find that your players are great at tossing you unexpected curveballs. By identifying and enabling a number of possible sub-goals, though, you'll help guarantee that there is enough meat beneath the surface of your adventure to help you deal with the unexpected.
When I ran an early version of this adventure, for example, one of the PCs tried to take the orc leader hostage and bargain for the party's freedom. Although I didn't anticipate the tactic while I was designing the scenario, I fleshed out enough details about the temple complex while preparing for the possibilities I did foresee to easily improvise and adapt.
In fact, the capacity to adapt is so important that you should make sure you're not falling into the trap of over-anticipating the PCs' actions while identifying your objectives. Beginning DMs often make the mistake of designing their adventures around a single, expected plan of action. Should the players start to deviate from this expected plan, these DMs tend to react by subtly (or not-so-subtly) "steering" the party back on course. Of course, this sort of steering inevitably frustrates your players and takes control of the game out of their hands. In general, if you can't look at your adventure and easily identify either several different ways the players might pursue your main objective or several different objectives to choose from, it's probably time to go back to the drawing board.

Fleshing Out the Challenges

Remember that the Fourth Rule of Dungeoncraft states that "Good adventures always challenge the players and their characters." After you've identified and addressed the various objectives the players are likely to pursue, it's usually a good idea to take a fresh look at your creation and make sure you are adequately accomplishing these aims.
"The Scar" looks like it has the potential to sufficiently challenge the players in a number of ways. First, the sheer number of potential approaches to escaping the temple promises to force the players to make a series of interesting decisions. Furthermore, several of the various objectives give the DM the opportunity to ask the players to solve puzzles. Using the mushrooms found in the fungus garden to poison the orcs' wine, for example, is a classic "item puzzle." We can also embed some interesting puzzles into the friezes that blanket the walls and into the various relics of the temple-builders that remain in the complex. Because the PCs aren't the only prisoners in the temple, we can also give the players plenty of opportunities for interesting interaction with the NPCs.
As far as challenging the players' characters is concerned, the fact that the PCs begin play without any weapons or armor should lead to some challenging combats. "The Scar" will certainly include some kind of mechanic that forces the PCs to make various saving throws or ability checks to avoid succumbing to weakness as their imprisonment drags on. Sneaking around the complex after hours and employing a few of the alternative escape routes should test the abilities of PC thieves. Making the most of the rudimentary spellcasting abilities at their disposal should adequately test the PC clerics and wizards.

Doling Out the Rewards

After you've completed everything else, take another look at your adventure and think about how you plan to reward the players for overcoming the various obstacles you'll place in their path. Obviously, treasure and magical items are the easiest rewards to dish out. Although the Monstrous Manual book gives you a good starting point for assigning treasure to the various creatures inhabiting your adventures, you should always modify these amounts based upon the circumstances at hand. If a monster is likely to be encountered when the PCs are in a weakened state (because there are other nearby monsters they might encounter first, for instance), you should increase the treasure guideline listed in the Monstrous Manual book by as much as 50 or even 100 percent. If, on the other hand, the PCs are likely to encounter the monster when the creature is at a disadvantage, you should lower the listed treasure by as much as 50 percent. Because the PCs are generally disadvantaged throughout the course of "The Scar," we'll need to be somewhat generous in allotting treasure to the orcs and the other denizens of the temple complex -- maybe the equivalent of 5-10 gp per orc. The fact that the orcs are in the process of looting the ancient temple neatly explains why they are so wealthy.
As far as magical items are concerned, my own rule of thumb is to include two or three "important items" per adventure, but to make all of these items particularly difficult to find or recover. Exactly what constitutes an "important item" depends upon the average experience level of the PCs who will tackle the adventure. For this first effort, I might include a single sword +1, a shield +1, and a little dust of disappearance. I chose this last item because it might play a particularly interesting role in the PCs' escape. To keep things interesting, we can supplement the important items with twice as many lesser items -- healing potions, minor scrolls, a dagger +1, or some arrows +1, and the like.
It's important to note that money and magical items are not the only rewards at your disposal. You can also reward the players with simple information. If you've been dutifully following the Rules of Dungeoncraft, you should know a number of interesting secrets about your campaign world by this point. If you've done your job well, simple clues pointing to these secrets and the occasional major revelation make for rewards that are just as satisfying as money and magic. In "The Scar," for example, we can begin peeling back the curtain on the "mind flayer conspiracy".