Decide what type of American Idol office pool you would like to run. You can use a points system to keep score of accurate predictions each week. Or, you can run an elimination pool where everyone stays in until she makes an incorrect guess and the last person left is the winner. Or, you can assign a contestant at random to each player and the person who has the American Idol at the end wins.
Determine if you want to include an entry fee and a prize or just do it for fun. A small entry fee and prize can be a great way to encourage people to join. Just be sure to make sure that running a office fee with money involved isn't against your company's policies.
Invite people to join your pool. Send out email invites or post a sign-up sheet in the break room or other community area. Be sure to allow at least a week before the start of the point of the competition with which you want to begin your office pool. Bring it up in conversation with your co-workers and encourage them to join.
Run the pool. Compile the weekly guesses on Wednesdays and the results on Thursdays if you're doing a pool based on weekly eliminations. Keep track of everyone's guesses and results to determine the final winner.