Well, I'm by no means an expert; but this has always worked for me. I lay the carpet in place and let the carpet settle, as you are doing- warm weather helps, but this time of year you take what you can get. A heat gun can help with any errant wrinkles. Go to your local home improvement store and get a GOOD quality carpet knife (not a utility knife. A carpet knife has a hook on the end of the blade) and a roll of carpet tape. Trim to fit with the knife and a GOOD pair of sheet metal shears (not snips, SHEARS- the scissors looking ones. They come in handy for trimming corners and edges). I usually mark the carpet on the bottom with a Sharpie where I need to cut, so it's easier to keep track of if I flip the carpeting up to cut it. I can't emphasize enough to make sure the shears are sharp and you have a fresh blade in the knife (I sympathize with the arthritis. That's why things have to be sharp- if you try to force things along, bad things happen...). Use the carpet tape to hold things in place as you go, as you inevitably move things around as you work, and you don't want your positioning to change. The tape holds things in place nicely, and you don't have a big fight on your hands if/when you remove it next time. One sweet trick I learned (I think it was here on FABO) is to use a cheap soldering pen to make any holes you need for seatbelt bolts, seat mounts, pedal brackets, etc. Melts through like butter and seals the weave so it doesn't fray... works slicker'n stewed prunes. Can seal edges with it too. Carpet goes under the sill plates, but you MAY need longer screws than what you had with the rubber mat). I would probably trim off the jute backing where it goes under the sill plates, though.
Good luck!
