I've installed ready made sets but I've gotta be able to make them fit better then the ACC one in my '56!
Start with a piece of paper. Figure out where you want to cut it to fit your floorboards so it will lay smoothly. When you are satisfied. Lay the padding in the car and cut it the same way. Lay the carpet in your car and do the samething. You will have to sew the carpet together where you cut it or you will have to bind it, or both.
Here is a really good link from gilgonzales on Binding carpet and laying pad. http://www.jalopyjournal.com/forum/group.php?do=discuss&group=&discussionid=2401
As stated, use paper to make a template. cut the carpet slightly oversized, trim to a perfect fit. I've only seen it done when a neighbor was doing it, but it looked pretty straightforward. He sewed it all together with dental floss. Said it was cheaper and stronger than commercial carpet thread.
Join the upholstery group and ask. http://www.jalopyjournal.com/forum/group.php?groupid=174 I know there was a really good tech on this in the upholstery group but I am having trouble finding it right now.
Did mine from butcher paper template I built. Then sewed it on the back side with carpet thread by hand. I sealed the seam on the back side with black RTV, and it worked great! I have no carpeting experience, so just did what I thought would work, and it did! Grand total for carpet, thread, needles, etc. was under $40.
dependng on the carpet a steamer like the cheapies you buy for steaming your shirts or whatever works well for shaping to odd spaces too.