Stand close to a wall and use either a small ball or a pillow between your knee and the wall. Standing on the outside leg, hold the ball or pillow firmly against the wall with the other knee - with it slightly bent. Slowly let the inside knee against the wall move downwards to your are tilting your pelvis down, then 'hitch'; your inside hip upwards as high as you can go without leaning outwards. Repeat this hip down then hip up action.
Try not to leaning into the wall or lurch away - you should be aiming to be upright as much as possible. If you don't have a pillow or ball - just have your knee against the wall and use a small towel to help it slide.