Decide which team you want to play for and research their walk-on policy. Bo Schembechler, the legendary coach for the University of Michigan, was famous for never cutting a player who wanted to walk on as long as he put in the required work. This doesn't mean all coaches share this policy, so if you're looking for your best chance, do your research.
Contact the coaching staff. If you are not an invited walk-on--that is, someone who didn't get recruited by the school but was still offered the chance to walk on--the best way to do this is to contact the athletic department and ask to speak with a member of the football coaching staff responsible for the teams recruiting. Make sure to say you are interested in learning about the possibility of walking onto the team.
Ask the coach questions about the team's sports recruiting and walk-on policy; this will give you important details about the program. Get contact information for members of the coaching staff as well as a schedule and a list of things you need to know before practice starts.
Complete any requirements given to you by the coaching staff, and prepare for practice and workouts to begin. Walk-ons aren't typically given a lot of opportunities, so be sure to take advantage of any you are given.
Show up to all team functions on time and work out as hard as you can.