an easy version of one-legged squats.

The weak link in this exercise is the knee.  Anywhere along this excercise, the hips and thighs may have comfort and power to spare - but at the same time the knee can be strained.  Be aware and respectful of that weak link, the knee, and don't use thigh-power to power your way through knee strain; its a big mistake.  Go only as far as is healthy for your weakest link.

The tendons and ligaments of the knee are slow to strengthen and, if injured, very slow to heal.  So, the strategy is to increase blood-flow to the knee and increase strength of that joint gradually, over the course of many practice sessions.  An attempt to rush progress that leads to knee strain (or more pronounced injury) will only set you much farther back from your goals.  If the knees feel strain, you've gone too far.

LUNGES

Lunges are a basic exercise that're often done with body-weight only.

AIRBORNE LUNGES

An "airborne lunge" is simply a one-legged squat; the back foot stays off the ground, yet available to assist with balance, weight, if needed.  There is no lunging forward, but more of slow vertical movement: down (to whatever level is comfortable), up, emphasizing integrity of form.  Inhale into the lower tan tien while descending, exhale while rising.  (Again, no need to go down all the way: don't strain the knee!)