Sit on the chair. Depress your shoulder blades and keep your chest open.
Keep your toes up. Your knees should be over your toes.

1. Engage your glutes and hamstrings. Slowly stand up. Your body (pelvis) should move forward over your toes. Keep your torso at the same angle.
2. Slowly sit down without letting your shin move back. Touch softly the chair.

A tip to engage your posterior chain: try to pull back your heels against the friction of the floor (without real movement).