I did the carpet on my old TriStar. I was able to remove the old carpet pretty much intact in sections and used that as a template for cutting the new pieces. As has been mentioned, doing it in a warm place is important -- hot sun even better! -- so that the carpet can conform to the contours. Measure twice, cut once.
"For every complex problem there is an answer that is clear, simple, and wrong." (Henry Louis Mencken)
2005 X2, Viper Red, MCX, Acme 1285, PPass, rear 750 sacs, KGB, IBS
(previous) 2001 X5, 1991 TriStar 190