I've seen a tutorial on it before but my google-fu is failing me tonight. BackpackingLight would like to sell you the info here.
I love my Thermarest Z-Lite for its weight and good insulation value. As a 5'6" guy it is a couple panels too long. So I simply tore two panels off one end. It made it small er in profile and the extra panels easily stack if needed. One other bonus is that the two remaining panels can be used for a seat rest or as a perfect foot pad in a hammock. And no resealing.
Long Signatures Rule!