Either have your bottom set of ribs removed, be insanely flexible, or somehow become a friggin zombie-ninja-person, decapitate yourself, and stick your head between your legs. So pretty much what everyone else said. There really is no other way to do it... Unless you're none of these and just really flexible/nuts.
Buy a sex toy or go find a partner to do it for you. Geez.
Buy a sex toy or go find a partner to do it for you. Geez.