Rubbing puppies is fun, 'cause they like it and you like it. If you do a good job, the puppy in question will probably do something really cute and nuzzle or try to get you to pet it some more.

[edit] Steps

Extend you hand, palm down.
Slowly lower you hand towards the desired body part of the puppy in question.
Begin moving your hand back and forth in a rhythmic motion.
You can move to other parts of the puppy, particularly ones indicated by the puppy through actions such as rolling onto the back (rub their belly)
When you and your puppy are satisfied, you can stop.

[edit] Tips

Rub whatever mood you feel in...if you're excited, rub vigorously, or if you're sad, rub slowly.
If the small, cute puppy in question appears sleepy, rub slower, in an attempt to coax the puppy into sleep.
You can use both hands if you feel like it.
If a puppy attempts to "shake" with it's paw, this may be an indication it wants you to rub it again.
Talking to puppies while rubbing them can be fun.
Give them a bone or other toy to chew on while you're rubbing them.
Sitting or kneeling or even lying down next to the puppy is comfortable.
Patting a puppy a couple of times when you're done rubbing it adds finality.

[edit] Warnings

DO NOT RUB STRANGE DOGS, PARTICULARLY IF THEY ARE GROWLING OR BARING THEIR TEETH.
It is not advisable to vigorously rub a puppy on or by an injury, such as a cut or recently added stitches.
Rubbing puppies can be addicting and VERY fun for both you and the puppy.