Flexibility challenge!

Hey guys! One of my flat mates challenged me to be able put my foot behind my head before March the 5th. With a bend back, I can put my foot on top of my head but I want to be able to do it perfectly. What stretches and tips can you guys give me? P.S. There's money involved as well. :D

Goofy triangle choke stretch( looks like a triangle choke with your foot in front of your knee instead of in the bend. in mastering the rubber guard). butterfly. double and single lotus.