Before you start a manual stretch, you may want to put some baby powder on your hands to help you get a better grip on your penis.
Here is one variation of manual stretching:
A hot wrap is a good idea before any workout, but especially this one as it makes the tissues easier to stretch. So make sure you perform a hot wrap before manual stretching.
Make sure the penis is totally dry before you start, to stop your hands from slipping.
The actual exercise is simple, it just involves pulling the penis. Grab behind the head with your strong hand (if you are right handed use your right hand, if left handed use your left hand), then use your other hand to strengthen the grip. Then pull outwards. The length of time you pull is up to you but if you are just starting it will not be much longer than 15 seconds, unless you have strong hands.
While pulling the penis out you should perform a 'kegel', especially if pulling upwards. You will actually feel a more intense stretch if you do this.
After every pull you should then slap your penis against your leg. I know it sounds silly but is necessary to restore the circulation.
After you have finished your workout do another hot wrap to reduce the chance of red dots appearing on your penis.
Variations: Try pulling in different directions - left, right, up, down, but remember when pulling left to perform an equal pull to the right to keep your erections straight. Also try moving the head in circles as you pull, just experiment and see how you can improve the stretch you feel.