What is the goal of manual stretching? To put tension on the penile ligaments.
The suspensory and fundiform ligaments as well as the ischiocavernosus ligaments are attached to the tunica which surrounds the corpora cavernosa. At the end of the penis the corpora cavernosa both attach to a triangular ligament that is attached to the glans (head).
To put tension on this chain of ligaments grasp the glans with an overhand grip (thumb toward the body) and wrap the first two or three fingers around the glans and place the thumb on the underside. Squeeze the glans (using a tissue, piece of t-shirt cloth or baby powder can keep your fingers from slipping) and pull in five directions: up, down, left, right and straight out. You can begin with 30 seconds of pull with a slight break then 30 more seconds. Do that routine twice and work up to two sessions of one or two minute pulls in each direction.
Grasping farther down the shaft can involve the skin which takes away some of the tension you want to put on the ligaments. It can cause skin growth (false foreskin for the circumcised) which may or may not be something you want to happen. It can also cause pubic hair to appear on the lower part of the shaft due to skin growth.
That’s one way. Others may have different suggestions.