To do a Foot Jam Endo you...
1. Begin riding at a modest speed.
2. Lift up either foot.
3. As the name describes jam your foot in-between the fork at a 50 degree angle.
4. This should lift up you back tire which creates a foot jam endo
Again make sure when you are jamming your foot in-between the fork that it is at a 50 degree angle as it makes the foot jam endo clean.
In the video below two kids show how easy a foot jam endo really is.