1. You use whatever skill you are using to aid your ally's attack. e.g. I wouldn't allow a player to aid an attack if they couldn't damage the target with said attack and the target knew this. It isn't a flat check.
2. It is a DC20 modified by circumstance/environments up to a maximum of +10. I also modify it the other way if sensible. A creature that is deathly afraid of fire is more easily distracted by someone waving a torch.
3. It has to be explained. There is no element where they don't explain it, in the case of hitting a creature in the ankle I would just get them to roll an attack normally and not give an aid bonus unless there was more thought / reasoning that went in.
4. I have no idea what you were trying to ask here.
Aid is useful, but not that great on strikes until later where it is more reliable or you are using a skill other than your attack modifier imo.
Keep in mind that it is a circumstance bonus, and doesn't stack with flat footed or allow flatfooted effects to proc off of it.
Yes, teamwork is encouraged in PF2e and math adjustment like this is very useful / appealing.
I don't think this is right. The penalty that is applied from being flatfooted affects the targets AC, while aid gives a bonus to the attack roll. It's not considered stacking since they are being applied to two different statistics.
Keep in mind that it is a circumstance bonus, and doesn't stack with flat footed or allow flatfooted effects to proc off of it.
You are right, I was tired and had just been talking about shield bonuses and take cover.
I have a few slightly different answers than Gleeful Grognard.
1) Say the plan is to aid an attack roll. I wouldn't require the Aid-er to use an attack to do this. I think that you could Intimidate an enemy just at the right moment to throw them off their game and hopefully allow your ally to sneak in an attack. I can see Acrobatics be used to try to trip up the enemy, or even Athletics to slam them with your shoulder or shield to throw off their defense. None of these things are particularly "injurious" to the target, but would reasonably create a small moment of opportunity for an ally to attack during.
2) The DC is 20, and personally I'm hesitant to adjust that too much. If something is easy or hard, a GM can adjust it by 2 either way, accordingly. Very easy or hard is 5.. all the way to 10. Personally I'd use those adjustments more for out-of-combat skill Aids. Say I'm aiding somebody in a climb check and my character is Expert in climbing... well, it's a bit easier for me to give good aid due to my expertise, so maybe I get a +2 to my Aid roll... Or, say the situation is that we need to create a scene that embarrasses the duke at a state function, and my ally plans on spilling a drink on them. I plan on aiding by grabbing the princesses attention and laughing loudly at the duke. That's probably pretty easy to do as there will already be a commotion due to the spilled drink... The key here is for the GM to call for an attack or skill depending on the description. So if they want to shove an enemy to aid an attack, you call for athletics. For example.
3) I don't think it's necessary to enforce TOO much description. I enjoy it when players narrate their actions cinematically, but it's often impractical to lean into that all the time. It can slow things down. However you don't always Aid either, so it's a special moment. I'd want a sentence at the very least. "I drive my shoulder into the enemy to throw off their defense..." "I point out that the handhold to their right is particularly good..." "I give the duchess a look, raise my eyebrows, and roll my eyes at the vizier to let her know that I think his idea has no merit..." I think that's fine RP, and it doesn't require a theater degree. That's probably just about right.
4) No, I put no limits on Aid that the rules of the game do not place. The player must spend the action to prepare to Aid, and use the reaction when the time comes. And a little RP is nice. At lower levels Aid isn't so great of an option. Figure a level 1 Monk wanting to use a well timed jab to aid an ally's attack will probably be rolling with a +7. They need a 13 in order to Aid.. not impossible, but not super likely either. And if they roll a 1-3, they crit fail - which can be bad. So... the math doesn't really WANT you to aid at that point. But by level 4, that Monk is rolling with a +10, and now it's better than 50/50 that they will aid and it's very unlikely that they'll crit fail. And it only gets better. By level 10 Aid is all but guaranteed. So, as characters get higher level, they get better at aiding one another, and Aid becomes a powerful teamwork tactic, and an excellent way to make use of your 3rd action in combat.... IF you don't have a pressing need for your reaction! So you will find that characters will still need to weigh their options, and they will either benefit from aiding, or else they will throttle their own use of Aid due to wanting to use their Reaction some other way.
And one more thing: Inspire Competence, the Bard Composition, allows the Bard to use Performance no matter the skill being used (but probably not Sneak or similar), and there is a nice thing where failure counts as success, BUT! Note that this only works on Skill checks, not attack rolls (as it specifically calls out "...to Aid your ally on a skill check of your choice..." So that is an exception to the rule that Aid is for skill checks and attacks. If there's no bard in your party it's not going to matter but I figured I'd point it out.
I have a few slightly different answers than Gleeful Grognard.
1. I wasn't saying that I would limit it to attacks, I was more saying that if someone was using an attack to aid I wouldn't allow it to work if they couldn't conceivably threaten the target at all or if the target had no reason to be distracted by the aider. I absolutely allow and encourage skills and other actions :).
2. Yeah I generally don't adjust Aid by too much either, and honestly it doesn't make a huge difference outside of early levels but I find giving a better chance at a critical success (given that it is just helping another player's accuracy a little) is rewarding enough to get people to use it if they figure out a weakness to exploit. It doesn't mean it is a common ocurrence that I give out circumstantial adjustments :)
3. This is probably where we differ a little, I am more interested in the why a player thinks something will work before the how. I feel like it is easier to tell a player "no that probably wouldn't work" to a why, than letting them roleplay a short bit and then rolling it back saying "no sorry I am not letting your roleplay work". So in my game it tends to be. I do give hints or advice regarding what would make sense if they are in the right ballpark though.
Also, I just want to say... God bards are such a great support class right up there with alchemists. Lots of love for how bards were handled in PF2e compared to the base chasis in PF1e.
1) I treat it as a normal attack roll with all the relevant modifiers.
2) At low levels, when the monster's AC is below 20, I have been setting the DC at the AC of the monster instead. Once ACs become higher I'll leave it at 20.
3) Up to the players. We tend to do descriptions for an entire turn instead of splitting it between actions.
4) Haven't had to yet. There could always be specific circumstances but as long as an attack will work I'll let Aid work. If someone is trying to Aid an attack with a different skill I'd take that on a case by case basis.
Whatever works for your group, I tend to err on the side of letting players attempt what they want and have a decent chance unless there's a clear reason not to.