If you ever had to fasten a mantel to a wall you may be a little overwhelmed with the best approach. A French cleat may be just the right trick to pull this off. A French cleat is simply two beveled mating pieces of wood or metal. One piece is fastened to the wall and the mating piece is affixed to your mantel back or whatever it is you are trying to attach. The advantage of using this approach is that there are no screws or nails that have to dealt with on the visible side of the mantel. All is hidden on the back side of the mantel. Follow these simple steps.

1. Create a recess. You will have to create a recess in the back of the mantel shelf to use a french cleat. The depth of the recess will depend on the thickness of the material used in the cleat. So for 3/4" wood a 3/4" recess is needed.
2. Screw the upper part of the french cleat into the top of the recess area of the mantel as shown in the image below.
3. Temporarily position the mantel on the wall that it is to be attached to and mark the top of the mantel shelf with a pencil.
4. Remove the mantel and measure the distance between the top of the mantel to the long point of the bevel on the rear of the mantel
5. Now attach the bottom portion of the cleat to the wall using your last measurement . The line of the top of the mantel shelf measured down to the short side of the bevel is the appropriate distance. See Below.
6. Now you're ready to hang your mantel.

Make sure to anchor your french cleat with the appropriate anchors. Wood screws are fine for wood studs and drywall but for a masonry wall, masonry anchors will be necessary.