From Reclined Hand to Big Toe, sit up and separate the legs out into a wide straddle. 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 you.
Listen to your hamstrings! Go down as far as you need to feel a stretch, but you don't want to feel pain. Stay for at least five deep breaths.