Make sure you can do threes comfortably on the jump in question and that you know exactly how much air time you have. After that just pop as in a three but set the rotation a bit harder. I usually set the initial rotation about 30% harder than in 360s and stay committed for a longer time in order to get the spin, but that's just my personal preference.
The main thing is not to panic, as you might feel like you're losing control when spinning more than a three. This is normal and you should just keep a cool head. After all, you're adding 50% to the rotation. I like to suck my knees up a bit and reach for a safety grab, but that's optional.
Spot the landing at 360 and then either open up or tuck for 540. You might feel tempted to put your hands down pretty quickly if you land front seat, but try not to. It's surprisingly easy to hold on to switch landings, once you get the idea into your head. A mental trick that helped me with this was to grip my poles harder than usual just before landing.
After a while you'll probably find that 540s are easier to stomp than 360s, since you have more time to spot your landing and make adjustments. Once you've gotten used to the trick, the rotation shouldn't be much more difficult than 360 on a jump of any size.
Send it, son!
