If you don't mind ruining the old carpet, it's actually somewhat straightforward. I replaced two pieces in the hatch / rear wheel well a few years back. Do it in the Summer or on a warmer day as that will ensure the glue is a least somewhat pliant. Be prepared to sweat and curse a bunch. Get a nice, sharp utility knife with lotsa replacement blades... As previously mentioned the glue is incredible and gums up everything. Try not to get the glue on your body! Remove everything that you can from the interior- front and rear seats, center console, etc. As you're pulling back the carpet, you can score the glue with the utility knife. Keep pulling the carpet and keep scoring the glue. The pieces will come out. For larger pieces like the main under-the-front-seat piece, you will probably have to cut the carpet in chunks. This will make it easier overall, but you do trash the carpet. Try not to sit or lean on an area where there is glue- you will stick to it and it will pick up dirt, and whatever else is on your part that touched it (yes, including your hair!).
Take your time and have plenty of your favorite beverage around... it helps to take breaks every now and then to relax your forearms and quench your thirst...
