Sit in a split squat with a half-butt on a chair in a narrow stance (it is more difficult if you sit on the middle of the chair).
Both of your feet should be next to the chair.
Keep your weight over your front foot.
Keep your torso fixed.

1. Pull yourself over your front leg.
2. Stand up over your front foot.
3. Bend your knees and sit back on the chair (preferably) in the middle.
4. Let yourself sit on the chair (relax your muscles).

The front leg is the boss, the back leg is the assistant.
You can decide how much the back leg helps and how much the front leg works. If you feel stronger, you can do it as a half-squat.