Replace the hinge screws on the bathroom door jamb with 2½-inch wood screws. Open the bathroom door and replace screws starting at the middle hinge. Remove one screw at a time and replace with the longer wood screw. Repeat for the top and bottom hinges. This will help the door to accommodate the weight of the mirror.
Close the bathroom door and have a helper hold the mirror on the back of the door. Position the mirror so that there is equal distance from both sides of the door to the sides of the mirror. Place a 4-foot level across the top of the mirror to level the mirror.
Trace a light outline of the mirror onto the door with a pencil. Set the mirror down and out of the way. Measure 2 inches in from the bottom corners with a tape measure, and mark with a pencil. Set one of the plastic mirror clips on the bottom trace line on the door. Measure 2 inches in from each corner on the top line, and mark with a pencil.
Measure from the top corners down 4 inches on each side and mark. Measure from the bottom corners 4 inches up and mark. Set a mirror clip under the bottom right corner where you have the 2-inch mark. Mark the mounting hole with a pencil. Repeat this on each measurement along the mirror tracing.
Drill holes for each mirror clip mounting hole using an appropriate size drill bit for the hollow door anchor. Insert the mounting screw into the face of the mirror clip. Insert a hollow anchor into the bottom-right and bottom-left mounting holes. Attach the bottom mirror clips by securing the screws into the hollow anchors. The anchors will expand over the inside of the door as you tighten the screws.
Set the mirror into the bottom mirror clips, and line the mirror against the original tracing. Insert two hollow anchors above the top of the mirror, and attach two mirror clips. Finish the project by inserting the side hollow anchors and installing the mirror clips.