You use the palm of your hand and push your glans into the shaft, as if you're trying to squash it shorter. The amount of force is up to you, I usually use enough to feel a burn in the tunica. The way I see it is if you don't feel it, it's not doing anything.
I've attached a beautifully drawn pic to illustrate (note that I had to include the [words=http://www.mattersofsize.com/join-now.html]MOS[/words] logo in case people try to start using on their sites)