The bottom turn is the first and most important manoeuvre every surfer should learn.
As you drop into the wave, the idea is to transfer your weight smoothly to the inside rail and turn the board along the wave. To do this, simply lean over towards the wave face, keeping your knees bent, until your board turns and sets a course along the wave face.
A good bottom turn converts the momentum gained from the take-off into the speed necessary to continue down the line, or into the next manoeuvre.