Assisted deep squat

Also known as: Jackknife squat, Supported squat.

Technique

Hold on to a stable object in front of you and bend your knees to squat down.
Go down in a controlled manner, without falling quickly into the bottom position.
Go all the way down until the back of your thighs are compressed against your calves.
Then extend your legs back to the starting position.

The feet stay flat on the ground, with your weight evenly distributed on them; do not let your heels come off the ground. If needed, you can elevate your heels with a board or another thin object to make it possible to go all the way down, without being limited by your ankles flexibility (and make sure to work on your Ankle dorsiflexion with other exercises).
Push your knees out: do not let them cave in when you go down, but pull them out with your glutes (butt muscles). They should be aligned with your toes when going forward.
Ideal feet width and angle depends on your body mechanics - play with different positions to see what works best for you.