Best three drills I use are the straight arm drop from an elevated bar onto a soft mat, focusing on straight elbows and heels lifting as a lever (so, straight body). Chin should be raised up, so the gymnast lands on their torso, not their face.
Second, on the floor, use a mushroom (boy's horse accessory) or other mat elevated. Have the gymnast lay on the floor with their feet on the mushroom (not legs!) and "sit up" so their nose touches their knees.
Lastly, on the bar, have them push to a comfortable support, chin up, and you stand behind the bar. As they go forward, you hold their legs, and help them feel the support on their legs and staying straight. It will hurt a few times, sorry to say, but they will get used to it and begin to understand the support they need.
After that, it's just practice. I usually put a porta-pit under a bar, and after sufficient drilling, I let them have at it, and if I see something particularly out of place, I pull them and work on the individual piece that is broken. Usually, it's from losing pressure on the bar (bending arms too early), or not sufficiently "sitting up" (or, more realistically, keeping their legs to the bar while rotating).
