I agree with what Chuck and Yeti said above.

I used a grinding jig from Midway but before I started chewing up a $30 pad I made a "trial pad" out of a piece scrap pine, just to make sure that I had the jig's angles set correctly.

Grind down close to your line and then finish up by hand with progressively finer paper for a really smooth surface on the hard pads. On the soft pads it doesn't matter as much, you can't get a smooth finish on them.

Dan