1. Sit on the floor, with your back upright, and the bases of your feet in contact with each other
2. Your knees should be pointing outwards and your core should be nice and tight
3. Take hold of your feet with your thumbs on the top side of each foot and place your elbows on the insides of your knees
4. Lean forward, gradually, at the hips until you feel a stretch in your hip flexors
The primary muscles stretched in the Butterfly are the Hip Flexors.