There is only one way to prepare for off-piste skiing and that is by throwing your self into it. Find easily accessible terrain, covered by fresh powder, if possible alongside a marked trail or between two marked trails. The terrain that you choose ought not to be too steep. It is important that you start out by practising powder technique on terrain that you feel comfortable with.
It is easier to ski short turns that long turns, and this is the technique you should focus on at first. Begin by concentrating solely on finding good rhythm. Rhythm is the most important tool in helping you to overcome the large amount of snow. Let your skis drive through the snow, do not try to move the snow itself. When your skis are completely covered in snow, they will not move from side to side but can move forwards easily. Therefore, let your skis build up pressure and use the short turn rhythm as momentum to start each new turn.