Replacing a mirror or cutting a mirror piece down to a different size can be accomplished at home with the proper tools and a bit of patience. It's not difficult, but you'll want to take measures to ensure your safety because you'll be working with a material that could cause injury to you or those around you. If you're making a small size adjustment of 1 inch or less or have a large piece to cut, it may be worth it to take the mirror to a glass professional--making those type of cuts is much more difficult.
Practice cutting mirror scrap pieces by following Steps 2 to 7 to be sure that you're able to complete a cut without error. Cut several times until you're satisfied with the outcome of your cutting. If you're placing the replacement mirror into a frame, keep in mind that the edge will likely be hidden by the mirror frame's edge, so a perfect finish cut is not mandatory.2
Lay out your newspaper or the drop cloth around the area where you'll be working. Generously cover the area where you are working--mirror shards can travel a long distance. It is best to have a wastebasket or small box nearby for the excess pieces. Make certain your work surface is completely flat.3
Put on safety goggles and work gloves before handling the mirror. The glass can be sharp and cause deep cuts, even without much movement.4
Measure the desired size of the mirror to be cut using a straight edge and a square. Using a square will insure that the corners are cut at 90 degrees. It's best to deduct 1/8 inch from the desired size to allow for easy fit into the frame. You don't want the mirror to fit in tightly; a tight fit can cause the mirror to break. Using a felt marker, draw this straight line onto the reflective side of the mirror. You'll be cutting the reflective side of the mirror.5
Place the straight edge on the line to be cut, with the straight edge resting on the good side of the mirror. Firmly hold it in place, or clamp it down to the table; you don't want the straight edge to move during the cutting process. If your glass cutter doesn't have a lubricant storage area, dip the edge in lubricant (sewing machine oil works well). Placing a fine layer of lubricant along the cut line can assist in a cleaner cut.6
Use firm, steady pressure to move the glass cutter along the marked line, keeping it tight to the straight edge. Start at the top edge and keep cutting until you reach the bottom edge. Don't move back and forth.7
Move the glass so the scrap area is hanging off of your work area. Using both hands, grab the scrap side of the glass about 1/3 of the way from the top and bottom and use firm downward pressure to snap the piece at the mark. Continue around all sides that need to be cut. Finish the edge with a mirror sealant or clear nail polish if desired.