Setup like a Heel Varial and go at a pretty fast speed. Not too fast. Then Pop down but you pop at the same time or even before you pop you should do a 45. That gives you a head start in doing the full 180. If your having trouble spinning the whole 180 still you can twist your back foot so that the heel is 45 degrees to the board. It's like a daffy stance. Instead of you back heel being 90 with the board your  back heel is 45 to the board. That way when you pop it helps spin the 180. Try to think of it as a normal F/S 180 but at the same time instead of dragging you do a Heelflip along with the 180. Try to stay over top of the board and don't be afraid to land it.

Good luck!

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