Installing a mirror flush against the wall and with brackets in the right places is important because mirrors can be heavy and are potentially harmful if they should fall from their mounted positions. A Mirror generally has two mounts on the bottom edge to hold the weight and two at the top to hold it firmly in position.
Position the mirror on the wall so that its centre is at eye height. Use a spirit level to make sure the mirror is straight and run a pencil along the bottom edge of the mirror, against the wall. Mark the centre of the pencil line. From the centre, mark out two-thirds of the distance to the edge on each side. For example, if the distance from the centre to the edges is 12 inches, make a mark 8 inches from the centre, on each side.
Hold the bottom mirror brackets against the two marks and mark with a pencil the location of the bracket screw holes, against the wall. Run a stud finder against the wall. If there are no studs at the locations of the screw holes, drill into the screw holes and insert a plastic wall anchor (anchors usually come with the brackets). Use a screwdriver to attach the brackets to the wall.
Lift the bottom of the mirror into the brackets and rest the mirror against the wall. Run a pencil along the top of the mirror, against the wall. Remove the mirror. As before, mark the centre of the line, and make marks two-thirds of the distance to the edge on each side of the centre mark.
Position the two brackets on the marks, and mark the screw holes on the wall. Drill holes and install the wall anchors, as needed. Install the brackets to the wall. Note that these brackets have slots that slide up. When the mirror is inserted into place, the slots are pushed down over the edge of the mirror, holding the mirror in place. Push the bracket slots up by hand so the mirror can be placed flush against the wall.
Slot the mirror into the two bottom brackets and hold the mirror with one hand against the wall. With the other hand, push the two bracket slots at the top down over the top edge of the mirror so it is held firmly in place.
After installing the brackets, use an eraser to remove all pencil lines.