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Mounting hooks are ideal for vanity mirrors that weigh 15 to 30 pounds (6.8 to 13.6 kg) and have a frame made of wood, metal, or plaster. The mounting hooks will ensure the mirror hangs correctly and is well supported on the wall.  Weigh the mirror on a bathroom scale to ensure you get mounting hooks that are strong enough to support the weight. The weight restrictions for the mounting hooks will be noted on the package. Look for mounting hooks at your local hardware store or online. Place 2 D-rings horizontally along the top of the frame. They should 1 to 2 inches (2.5 to 5.1 cm) from the left and right sides of the frames. Check that they sit evenly across from one another with a leveller, then attach them with a screwdriver. Make sure the straight part of the "D" faces downward. Some mirrors will come with D-rings already attached to the back. If this is the case, you do not need to put any more on the back of the mirror. Use the leveller on the top edge of the mirror to make sure the mirror is level. Make sure the mark is easy to see, as you will use it as a guide when you hang the mirror.  Putting the mirror over a stud in the wall is ideal, as this ensures the mirror is well supported. You can use a stud finder to locate a stud. If you cannot find a stud in the wall, you can still hang the mirror with wall anchors as long as you have a power drill. Lay the mirror flat with the back side up. Loop 1 of the mounting hooks onto 1 of the rings and place the back of the hook flush against the mirror. Take a ruler and measure from where the hole in the mounting hook sits to the top edge of the mirror. Do this on the other side of the mirror, looping the other mounting hook into the D-ring. Measure that side as well. The measurements should be the same for either side. Use a pencil or painter's tape to mark the spots where the mounting hooks will sit, using the top center mark you made earlier as a guide. Line up the hole in the mounting hook with the mark you made and then use the power drill to put the screw into the hole.  The mounting hooks should come with screws that fit in the holes. If you are not using a wall stud as a guide for the hooks, put a wall anchor in first before you drill in the screw. Most hardware stores will sell screws and anchors together so you do not need to install them separately. Loop the D-rings onto the mounting hooks on the wall to hang the mirror. Make sure the mirror sits flush against the wall. Place the leveller on the top edge of the mirror to ensure it is level as a final check. If you are worried about the mirror scratching the wall, you can put bumpers on the bottom corners of the back of the mirror. Bumpers are small plastic bubbles that have adhesive on one side and can be found at your local hardware store.

Summary:
Use mounting hooks if the mirror is large and has a heavy frame. Attach D-rings to the back of the mirror if it does not already have them. Place the mirror against the wall and mark the top center spot. Loop the hooks onto the D-rings and measure to the top of the mirror. Mark the spots for the hooks and attach them to the wall. Hang the mirror on the mounting hooks.