Sentient Items (Players)

Crafted items that gain sentience are given properties and stats as though it is a character; an alignment, mental ability scores, senses, a special ability, a purpose, and a personality to encompass it all.
Whether the item has attunement or not, and the type, is up to the item; as is the item’s value. Should you act in a way that the sentient item does not agree with, it can choose to inert itself. This does not force itself to be unattuned, but the item's magical effects will not apply, as though it were not attuned to. The only way to undo this is for you to roleplay with and appease the item.

Alignment

You cannot wield a sentient item with an opposing alignment. An item does not change alignments unless under very specific and special circumstances. It is possible to craft a sentient item of the opposite alignment, and being unusable for it.

Purpose

These are the reasons for the sentience's existence. It can vary between many things, but it always fits the item’s alignment and theme.
The number of methods to awaken a dormant item is equal to the number of purposes it has. The methods will be directly relevant for each distinct purpose.

Mental Ability Scores

The sentience has an Intelligence, Wisdom, and Charisma score, and all that implies. It makes saving throws if targeted by effects that require such saves. They are considered constructs for the purposes of considering immunity to certain effects.

Senses

The item will have some way to be aware of its surroundings and communicate. Such methods can include empathy, voice, telepathy, blindsight, lifesense, darkvision, etc. Whether the method of communication is limited to you or whoever is within range, or some other kind of factor, is up to the Crafting DM to decide appropriately.
Because a sentient item can be used as a second skill checker in exploration, then its senses will be limited by its rarity.

Skills

An item’s ability to use skills will be directly relevant to its senses. They will naturally have Perception, Insight and Investigation skills, but some other skills can be relevant, like deception, intimidate, any knowledge, martial, and so on. The rule of thumb is that a sentient item will not be able to make ability checks that require physical movement or interaction, like athletics and acrobatics.
For example, if an item can hear but cannot see the physical world, then it will have a perception for hearing, but not for sight, just like a blind character.

Special Ability

The item will have one or more abilities that are appropriately strong and can be used either by the wielder, or the item itself. The special abilites are more directly attributed to the item’s sentience and its purposes than the other effects of the magic item in general.
A sentient item will generally have a number of special abilities equal to half its purposes, rounded down to a minimum of 1.

Personality

The item has an ego just like a normal, self-aware character - its sentience. It will be congruent with the alignment, and at least considerate of its mental scores. The item decides how valuable itself is if it should require attunement and whether you are deserving to wield and/or attune to it.
For example, an item that would desire to not require attunement may have a sociable and perhaps flirty personality. A greedy or haughty personality would probably think itself worth a lot more than it actually is. A clingy and dependent personality may try to convince the player to unattune from other items. An aggressive and bold personality might favor fearless, melee combat-focused characters over the sneaky and spellcaster types.

Roleplaying the Item

Since sentient items are characters, then they will need to be roleplayed. If you wield, wear, attuned, or otherwise possess a sentient item, you cannot be the one to RP it.
The priority for roleplaying an item is as follows:

1. The RP DM in charge of sentient items
2. The Crafting DM that designed the sentience
3. Some other RP DM/staff

If you are in the middle of a game session, and if the above-mentioned people are not readily available to jump in and roleplay the item, the DM can roleplay it themselves like any NPC for the sake of time.
Generally, a sentient item's RP agency should be suppressed during a session for inconsequential things, like players just messing around. Note that this does not suppress ability checks or senses. DMs already have to deal with the players themselves, they don't need a 5th, 6th, etc. character to disrupt the flow or increase the time wasted to levity. However, if the story circumstances are relevant to the sentient item - usually determined by its purposes or personality - that is when it is highly encouraged to allow the sentient item to have agency.
Due to the potential for the running DM to have to roleplay the item, and also to be informed on when a sentient item will be relevant, it is required that the DM has the item's purpose and personality information available to them. Thus, it is required of you to inform your DM of a sentient item's presence, should you have one, at least by the start of a game sesssion.

Combat

In most circumstances, you are in control of the item. If you're attuned to it or wield it without a problem, then this much is assumed. Though the item may speak in the midst of fighting to taunt enemies or inform you of something, it does not have its own actions. However, a sentient item is able to use its own special abilities when the need arises, which is generally to fulfill one of its purposes, but only if it believes you are not working to fulfill those purposes yourself.
For example, if a sentient holy sword has a purpose to eliminate demons and a special ability that is effective against them, and you're fighting a group of enemies that includes a couple of demons and some irrelevant creatures, the item will most likely keep to itself and let you do all the fighting like normal. But, if you were to give some impression that you're avoiding the demons for whatever reason, the item might try to convince you to ignore other enemies in favor of the demons. If you ignore the item despite this, then it might force its special ability to be used against those demons without your consent.
But like any situation of conflicting priorities and personalities, there are nuances to social interactions. In the above example, if the item had a more aggressive or assertive personality, it might not even ask you to attack demons, it might use its special ability right away. If the personality was more accommodating, then it might be more patient with how you conduct yourself.
This is why you cannot roleplay your sentient item, because even if you act against the item's purposes, you might be tempted to ignore the consequences and metagame to justify yourself. And the complexity of social interaction is why the DMs who oversee sentient items or who designed the item have priority in roleplaying the item, and why its imperative a DM in a session is at least aware of the item's details.
When a special ability requires an action or bonus action to use, should the sentient item ever want to use that ability itself, it always has that option either right before the start of your turn or right after the end of your turn. Rather than having its own turn in the initiative, it instead has these two specific instances, so it could be described as acting on your turn in a way, but not entirely limited by it. In any case, due to the item acting in this fashion, it ignores any action requirements to use its special abilities, though it can only use them once for each specific instance.
Alternatively, if the special ability requires a reaction, the item can always use it whenever the ability would allow it. If the item does use its reaction to use its special ability, you cannot use your own reaction in response to use the special ability too.
Of course, the item using its special ability counts for use limitations. If the special ability can only be used once, then that use is gone until it refreshes.