A "J turn" is really just turning across the slope, until you basically go up, in a U shape...
Start going straight down, on a blue run. As you get a bit of speed, quickly look up to make sure nobody is coming and tip the skis to the right (or left, depending on which side of the slope you are).
As you edge the skis more and more, they will start to carve and turn across the slope - keep turning until the skis go uphill and stop. As they stop, turn around and start a J turn going the other way.
Here are some things to pay attention to:
Make sure you edge and carve the ski - this is above all a carving exercise.
We call this a J turn as opposed to a U turn because you always scrub some speed while turning and stop shorter going up the hill than where you started the turn, so it's never a complete U.
The point of the J turn is to allow you to explore the movements and carving edge engagement. The more things you do wrong, the less higher up you'll go on the other side, so that's a good gauge of doing things right. So play with movements (more on the outside versus both, edge quicker, grind versus float etc).
Do J turns across a blue slope. Don't forget to look up every time.
Explore how different movements and ranges of motion influence how sharply you turn and how high up you get (how much speed you carry):
Get into a contest with a buddy: who can get higher up the other end!
Look for smooth carving and carrying more speed up the hill, to gauge success.
You should progress from here to one footed J turns and then J turns with a very wide stance. Both very good if you have some trouble committing to the outside ski.
Other tipping and carving drills are: