Step 1: Materials needed
Just lay the pants over the damaged area, and cut around it. I like to leave about 2" of material past the edges of the hole on all sides. This allows some extra for the hem, plus it makes sure that when I sew it down, I'm sewing into good material, and not part that's about to fall apart. Next, make sure you have rounded corners. If you leave a sharp corner, it will tend to get caught on things, no matter how well you sew it down.
The other three things you will need are curved needles, ultra-heavy-duty thread, and some foam (I used a mattress topper). You will also need a sewing machine and regular thread. Oh, and you'll need a truck with a hole in the seat ;-)