A friend and I put new carpets into his car a few years back. We found that we had to trim quite a number of pieces, including the big piece in the back. The toughest - the two sill pieces that fit on each side of the car. These were an exercise in patience, and the carpenters maxim of "measure twice, cut once" only we went with "measure about six time, then cut once!"
We were able to use the pieces with piping.
My car (280 SL) with the original carpet set, fading and all was what we used as a template. The most difficult item, and one we set out to a pro, was the carpet on the two wooden covers behind the seats. The carpet on my car was steam-formed round the corner as there were no cuts in the carpet to make the angles.
Our trials and tribulations were chronicled in Pagoda World issues 29 and 30. Issue 29 is available for purchase, but Issue 30 no longer available.