Sit firmly on a chair and cross one leg over the other.
Rest one hand (or arm) on the inside of your knee then press your ankle down into the knee of the opposite leg by rotating at the hip joint.
You will notice that the knee wants to lift if you are doing it correctly & the hands are in place to prevent this from happening.
PRESS THE KNEE UPWARDS INTO THE HAND at a comfortable resistance (~20% resistance to start). Make sure your leg does not move and that you relax your head and shoulders.
Hold for 10-20 seconds.
Release and then bending forward from the hips, PUSH THE KNEE DOWNWARDS you will feel a stretch in your piriformis.
Hold this stretch from 20-30 seconds.
Swap legs and repeat.