University of Hawaii quarterback Bryant Moniz is a former walk on who is getting lots of attention around the country and there is talk of him possibly winning the Heisman.
Sam Bradford who played for Oklahoma and won the Heisman trophy was a walk on quarterback. This is an extremely rare feat, especially at this position.
Here is a couple of walk on players , Colt Brennan, Sam Bradford. Learn about the position you want to play. You should expect brutal practices, build your endurance.
The first thing you have do is search not all teams have walk on try outs. You will most likely have to go to a small school or not so great city and look at school. They have 3-5 day try outs.
Their are very few walk on quaterbacks that are successful. Even if the walk on is good the coaches will play their scholarship players first and stay with them as a matter of pride. Most walkons that work out are kickers as they are very plentiful and come from soccer leagues.
Jonathan Smith at Oregon State came in during the middle of his freshman year to lead the Beavs. Ended up winning the Fiesta Bowl with Chad Johnson/Housh/Dennis Erickson. Amazing story for a walk-on.