Relaxed drop of a box/height and then upon impact, ball of foot, jump straight back up. Spending as little time on the ground.
Now if your heavy, start at very low heights like 12inches and over time work up to 24 inches or so. Once you can squat at least 1.5 to 2 times your bodyweight work up to 36inches or so.
If you crumple upon landing then it's too high for your strength levels.
These are hard on the CNS as you go higher, and will delay recovery of training around the time you do them by up to 3-5 days. We are talking about impact forces of up to 10 or more times your bodyweight here
Drop jumps - are where you just drop down and absorb the force without jumping. These help build eccentric strength.
Anyway don't take these exercises lightly, more knee and related injuries from these exercises have occured because people don't have the required conitioning and strength base to do em. And if your overreaching or anywhere near an overtrained state stay away. And never do them in a fatigued state , especially at the end of a workout.
I would do lower impact plyos for a while before going near these.