Huskemaw has a good video of lapping and mounting. I use a hands weight on it and let the grit in the lapping compound do the work. You'll know what works and what is counter productive when you start.
Alderman, I let the weight of the lap itself (and the abrasive), do all the work!
Also, as I move the lap forward and back, I also rotate the lap from left to right and Vice-Versa.
The latter is done, to even out the wear on the lap itself.