The first step is identifying a slope that is safe to ski on as well as long enough to allow the cuts and extensions to be taken and performed without too much of a hassle. Ask around and talk to different skiers to get a good idea of where the best slopes are.
Begin your descent down the slope as you normally would but try to maintain a specific speed. You do not want to go into the cut too fast or too slow. The latter will result in you pretty much coming to a dead stop, which causes you to lose momentum, while the former will probably cause you to end up on your rear end but only after taking a tumble.
The trick is to shift your body weight towards the slope and put your weight on the leg in the direction you wish to turn while extending the other leg slightly to provide balance. This can take a little bit practice so remember to keep trying until you get it right.
You can also use your hand similar to how surfers do but do not put any weight on it, instead use it to provide some friction and resistance to help you cut tighter.
It is best to approach the turn keeping a 45 degree angle in your mind for the cut. As you progress and your skills are more refined, then more ambitious cuts can be attempted. Remember to take your time and do not rush things as it takes a fair bit of practice to do this the right way.