From the Chevelle Forum
Fit and install your carpet making all your cuts for seatbelts, shifter, console, anything else that is needed on you car.
Making the seat track cuts should be the last thing you do before trimming the edges and installing your door sills.
1. Locate your forward mount points for your seat(s) make a 1.5" cut left to right, about .5"aft of those mount points. Center of the cut should be center of the mount point.
2. From the center of the 1.5" cut,make a cut all the way back to the seam for the front to rear carpet.You will make 1 of these cuts for each forward seat mount point.
3. For the rear mount bolts repeat the cuts as above. Except the 1.5" cut will be towards the forward side of the mount bolts. Make the lengthwise cut meet the lengthwise cuts you made in the front carpet pieces.
4. Now from the under seat seam, roll rear carpet section aft and the front carpet section forward.
5. Install seat securing it to the floor with proper hardware and to proper torque.
6. Roll/Stuff rear carpet piece under your seat(s). Now do the front carpet. Front carpet lays over the top of the rear carpet.
Installation complete Seat plastic trim was removed for clarity
As far as installing the carpet I recommend doing it outside in the sun. This is perfect time of year. Doing it in the sun will make the carpet more pliable. If there are any areas the sun won't reach you can use a hair dryer to soften it up. A good sharp razor knife is mandatory.
I installed carpet professionally for 20 years so it seems easy to me. One tip I will give for cutting the edges is the following. When cutting the carpet to fit use a magic marker to mark the carpet backing.
For example you are trying to cut the carpet at the door sill. On the backing of the carpet place a dot with a magic market near the front of the sill, another near the middle of the sill and still one more dot near the rear of the sill. Place these dots where you want the finished carpet to terminate.
Then flip the carpet upside down by pulling it into the interior of the car. Slip a piece of plywood between the carpet on the floor and the carpet turned upside down. The plywood doesn't have to be that wide. About 1 foot wide is good. The plywood is used so you don't cut through to the carpet underneath. On the flipped over carpet take a straight edge and connect the dots made by the magic marker. Then run your sharp razor along the straight edge. This will make a very good precise cut and you can use that method around all edges. When you can't get the carpet to lay flat just put your magic marker dots closer together.
When/if you trim the sides, make sure you have enough carpet, but not the padding, going under the sill plates. I leave enough under the plates to run the sill plate screws through the carpet so it can't pull out. On the rear (aft) end, I wouldn't trim anything there, except for the rear seat cushion (front edge) slots/holes. The more carpet you can extend under the rear seat, the better.