If your riding normally then you can ollie off the tail or nollie off of the nose.
If your riding switch then you can do a switch ollie off the tail and a fakie ollie off of the nose.
( For the purpose of this the nose is whats facing downhill and tail back uphill not the actual nose and tail of the board)
Best way to practise is to do them I guess. Load your weight onto the foot your gonna be popping up off. Jump up off of that foot. Suck the knees up to level out the board. Straighten legs and then absorb the landing.
Theyre the basic steps to anyway of doing it. Just takes practise. You'll find that ollies and fakie ollies are easier as your popping in the style of your prefered stance (Even though your riding switch to fakie ollie the movements are the same as an ollie). Then switch ollies and nollies are harder as your moving your feet opposite to normal.
Practice make perfect, in your daily riding, ollie everything you can, even the shadows on the snow from the trees. The more you do it the better and higher your ollies will get.