Put on goggles and safety gloves before starting your project.
Use rubbing alcohol and a small towel for cleaning the mirror of dust and dirt particles.
Lay out a large towel on the work area (which must be a flat surface). This will protect the back of the mirror from scratching. If you want, you can cover the back using contact paper.
Draw the cut line on the mirror using a marker. It is very difficult to distinguish a marker line from its reflection on the mirror; therefore, you should stick a thick masking tape on the line to make it conspicuous enough to give a guiding ledge for the cutter.
Lubricate the glass cutter using oil.
Exert consistent pressure to score the glass while it is on the towel. Exert just enough pressure to make a thin, visible line.
Over a garbage can, break the mirror from the score line. This will keep mirror slivers off the towel, which can scratch the mirror's back.