You are definitely climbing the ladder of your skills if you are trying to learn 540 flips. These are semi-difficult to do because if you have a 360 flip under your belt easy, with a bit more effort you will surely be able to pull it off clean. It will probably take you a bit more time to get the right movement and to understand the front foot flick than with basic flip tricks, but sky is the limit and if you really want it, you will have it in either a few hours or in a few days,It doesn`t matter as long as you are having fun!
So, don`t go for 540 flips if you can 360 flip as it is basic to understand how to flick and how to use your body weight to land. Going from that, you should know how to set your foot position as it is basically the same as with 360 flip, but considering you want to push your board into 540 shove-it instead of 360 you will need to flick a way harder.
If you are comfortable with 360 flips enough, you can go directly from 360 flip to 540 flip but It is always good to try 540 shove-its first, just to get that feel of how much power you need to get 540 shove-it.
Set your feet the same as you do for the 360 flip but put your front foot a bit closer to the middle of the board or even closer to the back foot. It will help generate more power to the flick. Try to keep at least half of your front foot on the board. If your front heel is too much off the board, the board will go into double flip instead of 540 shove-it. To send your board into 540 shove-it, use your back foot to scrape slightly harder than at 360 shove-its or 360 flips.
With a little effort you will soon figure out the moves and once you flick the board into perfect 540 flip rotation just go for the landing the same way as you do for the 360 flip.
Putting it all together, 540 flip is a great elevation from 360 flip and it shouldn`t be too hard to get, but can still cost some nerves. Take it easy and remember this can be a nice trick to give the letter to your buddy at the game of skate!