1. Go to the ride and look for the overhead signs "Fastpass Distribution" where there will also be the time shown to come back and use your fast pass.
2. Insert your park pass into a machine at that location The machine is about the size and shape of a steamer trunk standing on end, perhaps a little longer and skinnier.
3. The fast pass should come out of the machine, if not, try turning your park ticket upside down and inserting it again.
4. Be sure to take both your park ticket and the fast pass with you. Count your park passes before leaving if you are getting fast passes for the whole family.
5. At the time period shown on the fast pass, come back to the ride and look for the "Fastpass Return" entrance. You may have to squeeze by other guests waiting for their fastpasses to mature.
6. Note: For the most part you may not get two fastpasses in quick succession*. While waiting for your come back time you can go have lunch, or wait in line using the "Standby Entrance" for the same ride (if you want to go twice) or for a different ride.
7. Note: If nothing came out of the machine the first time and you finally cannot get the machines to work for you, look for a cast member in that area who should give you a fast pass for that ride. The time delay before your next fast pass somewhere else, up to two hours long depending on park crowding, may or may not have been imposed on you.
Disney hints:
http://members.aol.com/ajaynejr/fastpass.htm
* Lights Motors show at MGM Studios is an exception, you can get a fastpass for it about 10 minutes before or after getting another fastpass.
Caution: If your park pass does not come out of the machine, hold your hand over the slot and wait (or yell) for help.