The first step to an extension is finding a line down the slope. Look for a slope that is open and does not have too much traffic. Chances are that if this is your first time trying extensions you will be falling down a fair bit and it is best not to injure anyone else in the process.
With a line in sight, begin you descent down the hill. Before trying to begin the extension try and maintain a certain speed. There will be a fair amount of deceleration during the start of the extension so you do not want to be going in too fast or too slow.
Also, do not try to cut at too tight an angle but make it a gradual turn. In order to do this, stick the leg opposite to the direction you wish to turn out. This means if you want to turn left then stick your right leg out slightly and if you want to turn right then stick the left leg out.
Shift your weight on to the leg that has not been extended and tilt your body in the direction you wish to turn as well.
When you wish to turn in another direction, remember to keep your balance as you shift your weight once again. It should also be pointed out that the sudden change in direction will cause a tad bit of deceleration so make sure you are prepared for the drop in speed.