Kneel down, with the tops of your feet flat against the floor. (It's a good idea to perform this exercise on a soft surface). Sit back on your heels, with a tall posture & neutral spine.
Inhale & brace your core. Keep your back flat, fix your gaze forwards, & explode upward powerfully, slightly extended the hips. Bring your knees up, and drive your feet into the ground, landing in a squat position. Exhale, continue driving the feet down, extend your knees and hips, to come to a tall standing position.
Carefully return to the kneeling position and repeat for repetitions.
If you do not have adequate mobility or strength to keep a good posture in the bottom of a squat, work first on easier progressions.