You can help someone achieve success on a skill check by making the same kind of skill check in a cooperative effort. If you roll a 10 or higher on your check, the character you're helping gets a +2 bonus on his or her check. (You can't take 10 on a skill check to aid another.) In many cases, a character's help won't be beneficial, or only a limited number of characters can help at once.
In cases where the skill restricts who can achieve certain results, such as trying to open a lock using Disable Device
, you can't aid another to grant a bonus to a task that your character couldn't achieve alone. The GM might impose further restrictions to aiding another on a case-by-case basis as well.