It's free.
Just take your ticket that you used to get into the park and put it into a FastPass Machine.
On the FastPass ticket you will have a time to return to the attraction/ride (11:10am-12:10pm). What they don't tell you is you can return anytime after 11:10 am and use the fastpass. The CM's could complain or say no to the most popular attractions (TT, Soarn' and TSM) if you show up later than 12:10pm, but most let you through without any problem.
At the bottom of the FastPass ticket there is another time-it is the time that you can get a new fastpass from another machine.