The Box Squat auto-corrects your technique plus takes pressure off of your back & knees making it easier to recover from. It also teaches you how to properly sit & stand. I love using an adjustable aerobics step with risers for micro-progression. Add a mini-band wrapped above the knees to increase lateral hip activation & prevent inward valgus knee collapse.
The ideal squat depth depends on your individual hip anatomy, limb length, body size, injury history & goals. Thighs parallel to the floor (or slightly above or below) is the perfect functional depth for most people. If your goal is to go ass to grass, gradually lower the box height over time to get there safely. If you have a history of knee pain or you want to stress your glutes & hamstrings more, keep your shins vertical to the floor. If your goal is max quads activation, use a more upright trunk & forward knee position.
You can increase the difficulty by doing goblet or double dumbbell front squats. You can also add jumps from a dead-sit position for lower-impact jump training. Plus you can do them single-legged, just start with a higher box than normal. Once you’ve perfected the box squat, you’re ready to squat in free space. Try doing 10 reps every minute on the minute for 10 straight minutes. I once did this for 30 minutes straight with a 100-pound dumbbell and it changed me forever.