What do I have to sign to use power of attorney?

You don't have to do anything. If you want to sign their checks, just bring the POA to their bank. If you want to make trades on their broker accounts, just notify their broker and send him a copy. If your parents are competent, you ought to alert them what you are doing. Keep in mind you are a fiduciary and everything you do has to be for their benefit or you could wind up with legal problems.