remove basically everything that's removeable from the headlight. Unscrew all screws. Preheat oven to 220 degrees. Use a cookie tray or something flat and big enough to hold headlight. Cover that in tin foil so you don't mess up the tray or make it smell like headlights after. pop it in there (do 1 at a time) for about 10 minutes. anywhere around that time the glue gets soggy. underdo it and you'll have to rip the 2 parts apart which is bad and can damage them. overdo and you might start melting the clear part. its a balancing act.

when you take it out, WEAR GLOVES. soft leather or vinyl ones will work. pry apart. you MAY need to use a flathead to "help" pry them apart. be very careful cause metal can seriously mess up the plastic, so don't just have at it. be very patient, HOWEVER if you take more than about 30 seconds to pry apart it will start hardening back up and make it a nightmare. if that happens, pop it in for another 2-5 minutes and get it back up to temp.