Idk about flat, but for 180 whips, i spin and whip at the same time and right when the whip gets around pull up to catch it. for 360's just turn a little before you jump and huck it.
fs 360s are harder than backsides, but the main thing is to get the feel of fs 180s and really quickly rollout. This helps you to get used to not really being able to see where youre landing. I recommend learning em on a rug or someshit cuz youll fall on your ass alot hahah