I think everybody should know how to do a good hockey stop; I'm sure it's saved my life at least once.
Practice on a moderate slope with you skis 90 degrees to the fall line. Start by standing still, not going anywhere because your uphill edges are holding you back. Now tip your skis so they are more flat to the snow and not having your uphill edges dig in; you start to slide. Tip them back up, digging in your uphill edges; you stop sliding. Play with how putting more wieght on the tip or tail spins you around. Control it. Play with how more edge slows down more and less slows down less. Notice how hitting a bump with one end of your skis spins you around like a top. You want to avoid that at speed.
Lesson 2: your skis have edges that are beveled at the bottom, so the ski can easily slide around. If you angle it too much the edges will dig in. You use that when you slow down in the above exercise, and when you release the edges to slide. If you turn them too far you catch the other edge. That's called catching an edge. You don't want to catch an edge. On groomed snow you can easily release all the edges and pivot the ski around untill it is 90 degrees to the fall line, and then dig in the uphill edges. Sometimes it's easier when combined with upweighting. On deeper snow skis don't like being pivoted without a lot of upweighting, and even then they need to get an uphill edge near the tip digging in to pull them around to 90 degrees.
Now go practise.