The 540 kick causes a lot of problems for people who are confident with the 360. This is often due to starting with the opposite leg forward to the 360.
As backwards as it may sound - practice the 180 kick: this will get you more used to jumping with the kicking leg at the back.
Progress that to a 360 reverse turning kick with the opposite leg. This will get you more used to having the backwards facing part during the jump (whereas 360 kick the backwards facing is first). If you struggle with the 360 reverse turning kick then practice a 360 back kick first.
Building enough of a jump to manage a 540 spin is the next trick. If you are doing ITF style in Choong Moo you do a standing 360 degree jump - so we get used to these. To build up to the 540 kick, practice a 540 jump (if you can do it standing then that's amazing - most people will need to be moving forward to get that spin). This will seem much more difficult than the 360 jump - part of this is that we start the spin on the floor (maybe get as much as 90 done) and finish the spin with how we land (possibly another 90 degrees here) for the 360 this means that we can get away with only spinning 180 degrees in the air - meaning that for the 540 we actually have to spin roughly twice as much.
Once you can jump 540 and land comfortably then 'just add the kick' to the end of it. Again: many times we see spinning kicks the kick itself is not technical. We generally aren't going to be using this for self defence or breaking so don't be too worried if the kick is more of a crescent.
Don't over do practice of these kicks - they can be fun and fantastic for demos and showing off - however, they aren't useful for self defence and landing them can be a nightmare for your joints.