Stand with Neutral Spine, Butterfly and Core engaged.
Balance on your right leg and, keeping your knees roughly aligned, lift your left foot backwards. Take hold of your left foot in both hands and gently pull it upwards towards your bottom. If you can't balance on one leg unaided, pull the foot with one hand - the same hand as the leg you're stretching - while supporting the balance with your other hand, i.e. on the back of a chair.
Push your hips slightly forwards and hold the stretch for 10-15 seconds. When you feel the muscles relax, gently increase the stretch and hold for a further 10-15 seconds.
Release and repeat with the other leg