540s in the halfpipe aren’t actually quite as difficult as they sound, particularly as you are able to take off facing forward and re-enter the pipe still in that forward facing direction. Saying that, backside 5s are probably a wee bit tricker than frontside 5s, as achieving pop off your heelside edge on the backside wall is definitely more challenging than springing off your toes on the frontside wall. As Torah Bright explains here, after a slightly odd but rather amusing intro, it’s all about initiating the rotation quite aggressively with your front shoulder and committing to the spin with your head as well.
Get backside spins on lock off little sidehits on the way to the pipe, practice sending straight airs on the backside wall and then try combining the two sets of skills! With a bit of practice you’ll be sending ’em like Torah here in no time.