Measure the location of the light switch on the wall. These dimensions will give you the exact size for your cutout.
Measure the distance from the edge of the tile on the wall to the outer edge of the light switch on all four sides. Use these measurements and mark the location of the cuts on the tile.
Mark the tile on the front and back with a permanent pen where you will be cutting it. For instance, if the light switch is a rectangular shape that measures 3-by-5 inches, mark a rectangle on the tile with the same dimensions.
Place the piece of tile on two wooden blocks for support.
Drill a hole in each of the four corners with a glass-tile bit. Turn the tile over to the backside.
Attach a carbide-grit blade to a jigsaw. Place the blade of the jigsaw in one of the holes and cut to the adjacent hole, following your line.
Turn the saw at the corner and continue cutting until the section of tile is no longer attached.