Sit down with your feet about three to four feet apart (not as wide as you can go). Reach back with your right hand and move the flesh of your right bum cheek away from you, 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 begin to fold forward at your waist, sliding your hands on the floor in front of your head.
Go down as far as you need to feel a stretch in your hamstrings, but you don't want to feel pain. Hold for five breaths.