Work on this for about an hour slowly increasing the amount you let your skis turn before you stop. You can really start slowly and safely, let you ski tips point progressively further down the hill working at a pace you feel comfortable at. If you start to loose it just dig your edges in and you will stop. If you really loose control (which you shouldn't) remember to just curl up into a ball and sit down on you bum. It ain't pretty but it works!
The plan is to build up your technique by progressively letting you skis point down the hill by say 20° then 30°, 40°,50°, 60° and so on each time finishing with a stop until you finally are able to hit the magic 90° ( straight down the hill) and then stop again.
Try to feel as if you are lifting your heals a little when you start your stop and then rotate your ankles quickly into your stop. This will help you come around more quickly and be able to deal with steeper terrain.
Once you are confident at calling the hockey stop on your demand and you can let your ski tips slide all the way into the vertical (the fall line). You've cracked it!
This is a huge step forward as most intermediate skiers can’t do this even after a few years, this is due to an unnatural fear of having the skis pointing down the hill. As soon as their skis are pointing down the hill warning bells sound and guess what, out comes the snow plough. If only they could stop like we can, on demand with the hockey stop. Congratulations you have know learned the hardest part about skiing.