Decide on the dimensions of your new piece of carpet. Record the size of the area to be covered and add two inches to each side--this provides a helpful margin of error as you cut.
Turn the carpet over so the material faces downward and the sturdy backing faces upward.
Use a snap line tool or a bright piece of chalk to make cut-guiding lines on the carpet backing. To use the snap line, hook the end of the line to the backing's edge and release the appropriate amount of slack. Lift and release the line so that it snaps onto the backing, leaving you with a straight and visible cut line.
Put on a pair of thick work gloves. The carpet backing may be tough to cut, so wear a pair of appropriate work gloves while handling the utility knife.
Place the knife blade at the start of the cut line. Press firmly into the carpet backing with the blade and begin to cut using steady, single strokes.
Repeat the above step for each cut line. Clean up any jagged or roughly cut edges with the utility knife once finished.