Get your speed, when taking off the ramp, slowly rotate so your facing where you came from, it is a rickety landing so make sure you brace from it. When you are going backwards, some bmx's pedals will go backwards but on a lot they will not.
This part was trickiest for me and my friends, i've only ever pulled off a 180 off a jump once, i tried to do a sort of reverse skid. I twisted my handlebars to face the left and slid it round to face the i was originally going, but my mistake was not pedalling after i turned and my momentum took me backwards into a half-pipe...
Lol i'm rubbish at explaining, try looking for a video of someone doing it.
it aint hrd u got learn it on the grond and dnt b***h in the airWall jumping sport, what's it called?