Basically 3 ways to do these depending on what you like/what skis you have/where you are.
Jump method, most often seen in Nimbus in pow:
1) Rock back a little and jump 180 (ish).
2) Land with your weight right over your noses at around 180.
3) Continue to rotate your head looking for 360, when your head gets to 270 I tend to pop a little off the nose and start to bring it back round.
4) Try to land in a roughly centred stance, generally the first few times you either catch your tips or land really back seat depending on how hard you throw it.
Nosepop-Twisty Method, generally the easiest, also doable on stiff skis:
1) Rock back to get a little extra force, nollie but force the noses down, start looking over your shoulder and throw the 3 hard. Tails should be off the ground but noses still in contact.
2) Do not stop the rotation, if you do and your on stiff skis you aren't really flexing, you're gonna have a bad time!
3) At around 180 you generally leave the ground with all contact points, sometimes you need to push of the nose again, keep looking for 3. Land.
(This is more or less a nollie 360 but works on stiff skis if you lack the legs to flex them, this is a quick rotation, if you try these too slow or stop looking, you'll just do a nollie 1).
Slow and 'steezy' method, requires skis you can flex the shit out of but these are my favourite:
1) Carve in (my favourite way) and push in to nose press position, you need to get a good press or you'll catch an edge.
2) As you carve, slowly start the rotation with your head the skis will follow. You can stall out the butter at 180 (switch nosepress) or continue to 270 in the same press position on these, as long as you have skis well flexed. An edge detune in the tips can help on these.
3) Once you reach 270 (just before really) you need to pop a little off the noses to avoid catching on the last 270!
(These should be very doable on groomed on Opuses, just take a little practise, watch Max Hill).
Hope that all makes sense.