Begin by lying on your side with your upper body perpendicular to your legs, grab your penis and pull it towards your anus. Tuck it between your legs and then close your legs. Try to straighten your upper body. You should feel a good stretch!
Remember... after about 5 minutes, you MUST check your glans to make sure you are still getting proper circulation. You shouldn't perform this exercise for 15 minutes straight without checking "downstairs"!