I just bought a used truck in great shape, except for a nasty hole in the bench seat. Thankfully, that's pretty easy to fix. Of course, this isn't for show cars, but a work truck doesn't need to be pretty; just functional.
Step 1: Materials needed
First, you need some material to to use as a patch. Denim is my favorite patch material, because it's strong, and it's easy to get. Everyone wears jeans, and they all wear out eventually. When they do, save them; they may have holes in the knees, but there's still plenty of good fabric there for patches. This patch came from the back of a leg from an old faded pair I stopped wearing months ago.
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 ;-)