it's just like doing a shifty 1 or 3. jump and turn your shoulders like you would for the spin you want but keep your skis straight downhill. tap whatever you want, then whip your skis around to match your shoulders.
ya'll gonna hate me for saying this. But this is a big thing thats wrong with skiing. You don't need to ask for advice on something like this. Snowboarders started doin these years ago, mostly as an extension of style, to ad a personal touch to their tricks. Let it flow, mess around with it, nobody should be telling you how to do them.
It helps to start the spin out earlier (like a foot or 2 before the end of the rail), then set the spin a little harder, then simply tap it on the way out (do it REALLY quick at first, then you get it to look a bit better).