I basically did what Eugene did with my Thermarest. First, though, I looked at it and decided what length I wanted it to be as a torso length; which, for me was to the bottom of my butt cheek. YMMV. I made a little tick mark with a sharpie. Then, without cutting it, I folded it into 1/3s and 1/4s (again YMMV)and held it over the pad pocket of my pack. If I were using inside my pack and not the pocket, I suppose I'd fold it into 1/3s or make a cylinder out of it. The number of folds might also depend on which pack and which pad you're using as well as the width of the pad pocket. I think it was a GG Murmur pack the first time I did it. I did the same with my recently acquired ULA Conduit. Once the folds were even, I marked where I wanted to make my cut. I came to within about an inch or so of the length I wanted it to be as a torso length pad. The inch one way or another didn't make a difference in the comfort. I pretty much got what I wanted and I've always been happy with it. It probably took about 10 minutes give or take. Again, however, I've only done it with a Termarest.
BTW I'm 5' 9 1/2"