Lie on your back and cross one ankle over the opposite knee.
Rest one hand 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 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 repeat 2-4 times on each leg.
You can also perform this seated.