Borrow, rent, or buy a powder specific snowboard. Powder boards tend to be directional with a wider nose than tail. They will also tend to be a rocker camber.
Set your snowboard bindings as far back as possible, while maintaining a comfortable stance. (This will help the nose of your snowboard stay above the snow).
Find a slope that is steep enough to get some speed with out a flat runout. (It is hard to snowboard in flat powder conditions you will get stuck and frustrated)
Point your tip down the hill and lean back. You want your nose to be above the snow while your tail is gliding and turning in the powder. (For more advanced riding try gradually applying more and less pressure to your tail, in a slight bouncing motion to gain more speed)
Get on the chairlift and repeat steps 1-4 as needed!