Measure the carpet to see where the cut needs to be made.
Turn the carpet over or fold it back to see where the rows are. The rows are the areas between loops in the carpet. Though you can't see them from the top of the carpet, on the back you will notice "ridges" and "valleys". Cut along a valley line to minimize fraying.
Place the “mouth” of the cutter over the carpet so the blade and the “nose” of the cutter are between rows of carpet loops. You may find that it is helpful to "mark" the line you will be cutting by separating the carpet loops by running a pocketknife along the line you will cut. Most loop cutters are configured so the blade cuts through the bottom, or back, of the carpet.
Push the carpet trimmer forward using your dominant hand. Keep your non-dominant out of the way of the carpet trimmer for safety purposes.
Check periodically to make sure the nose of the cutter is still between rows.