Start slowly and increase your speed – later without pedaling. Through pulling and pushing with arms and legs while riding over the berms and rollers you can get faster and faster.
If you push and pull powerfully, you get really fast. Keep moving your hip, legs and arms and keep the rhythm. To turn in the berms & banks bend arms and legs and go down to find a good balance point.
If the track is well designed the rhythm will come automatically after a while. After some practise you can jump over the rollers and surf the dirt waves properly!