It's up to you whether someone's preparation is enough to let them
Aid an ally. The preparation should be specific to the task at hand. Helping someone hold a lockpick steady might be enough preparation to Aid an attempt to
Pick a Lock, but just saying you're going to “encourage” them wouldn't. Second, the character who's attempting to Aid needs to be in a proper position to help and able to convey any necessary information. Helping a character
Climb a wall is pretty tough if the character a PC wishes to Aid is nowhere near them. Similarly, a character usually needs to be next to their ally or a foe to Aid the ally in attacking the foe. You'll also need to determine how long the preparation takes. Typically, a single action is sufficient to help with a task that's completed in a single round, but to help someone perform a long-term task, like research, the character must help until the task is finished.