After marking the positions of all studs, use a long, straight-edged board (the same height as the walls) to check the wall surfaces.

Hold the same or a different straight-edged board horizontally against one strip, and if necessary, slide thin wooden wedges behind the strip to straighten it so that its entire length touches the board.

To prevent kickback, use guides, either homemade from straight-edged boards or those available as saw accessories, to keep the saw moving in a straight line.

To minimize kickback, use guides to keep the saw moving in a straight line (the guides can be homemade from straight-edged boards, or they can be store-bought as a saw accessory).

To guide the saw in a straight line if only a few cuts are necessary, clamp a straight-edged board to the panel parallel to the line along which you wish to cut.

To check the wall, use a straight-edged board longer than the mirror is wide.

If only a few cuts are necessary, the saw can be guided by a straight-edged board clamped to the panel, parallel to the cutting line.

Use a carpenter's level or a straight-edged board to make sure shimmed strips are flat in all directions.

To check for those, place a long straight-edged board on the floor in as many different places and at as many different angles as practical.

Use a long level or a straight-edged board to make sure that the strip is flat.