Lie on your side with legs straight and feet and knees together. Support your head with your bottom hand and use the top hand for support.
Take the top leg forward and place on top of a support such as a foam roller, yoga block or roller up towel. This helps to keep the hips in line a little more.
Lift the lower leg away from the floor. Hold at the top and then slowly lower. Do not rest at the bottom but raise back up again.
Ensure that you do not rock forwards or backwards at all (which means that you will probably not need to lift your leg as high as you think you do).
Also point the toes down towards the floor slightly.
Your goal is to complete 15-20 reps with relative ease. Build up to this gradually.
When you can complete these then add some resistance using a mini band.