Sitting on the ground with one leg in front, bring the other foot in to touch the inner thigh. Reach across with the opposite arm and hold onto the knee or ankle, to give some resistance to your stretch. This keeps your hips square and allows the stretch to get into the belly of the hamstring. Hold for 10-20 seconds and breathe deeply. Ideally a hamstring is better stretched with the knee bent, so try both bent and straight to see which position gives you an adequate stretch.