From Butterfly, extend your legs out wide. Reach back with your right hand and move the flesh of your right bum cheek away from you, and then do the same with your left. This will help your pelvis ground more firmly so you can stretch your hamstrings more effectively.
Sit tall, reaching your head away from your hips, and draw your belly and ribs in. Keep that length as you fold forward at your waist, sliding your hands down your legs or on the floor in front of your head.
Go down as far as you need to feel a stretch in your hamstrings and inner thighs, but you don't want to feel pain.