Place an old rug or cardboard pad on the floor. You will use its measurements for your new rug.
Lay the carpet remnant face-down on top.
Measure the carpet using a straight edge ruler. Mark the measurements on the back with a pen.
Weigh down the carpet remnant with a heavy object. If you don't have anything to weigh down the carpet, you can kneel on the carpet to hold it down.
Lay the ruler along a line, and draw the blade of the utility knife or carpet cutter along the edge. If you are cutting from the edge of the remnant, start your cut a couple of inches from the edge. Slide the blade against the ruler to cut along the marked lines. Repeat until the remnant is trimmed to the proper size.
Apply carpet edge sealer to the edge of the new rug. A carpet edge sealer keeps the edges of the rug from unraveling. Follow the manufacturer's instructions for mixing and application of the sealer.