You want to make sure the mantel is level not only side to side, but also front to back. Use a torpedo level to check the level front to back. [1] X Research source

One way to measure the cleat is to fit it into the back of the mantel like it will be on the wall. Use a measuring tape to measure the length from the top edge of the mantel to the bottom edge of the cleat. Place the measuring tape along the line drawn in step 2, and then use the measurement and draw a new line below the first. For example, if the length from the top of the mantel to the bottom of the cleat is 3", measure 3" down from the line on the wall and draw a second line. Another method to measure the cleat is to measure the size of the inside of the mantel from the top of the shelf to the edge just above where you will place your cleat. Then, measure the length of the side of the cleat you will affix to the wall. Add those totals together. For example, if the measurement from the mantel shelf to the edge is 2" and the length of the cleat is 1-3/8", then your measurement will be 3-3/8". Draw a line under the line on the wall using your measurement.

Measure the size of the mounting boards against the new markings on the wall, and use the saw to cut the cleats to size. The top cleat should be one foot shorter than the shelf. Dry-fit the mounting boards in the mantel. The top cleat should be fitted first, then the two legs. They should fit snugly together, but don’t have to be fitted perfectly. Make any adjustments to the length of the cleats if you need to.

Wall studs are meant to support and hold up dry wall on interior walls. [3] X Research source When you are hanging heavy objects, such as a mantel, you want to make sure that you hang the object on the studs. The easiest way to find a wall stud is to use a stud finder, which can be purchased from a local home improvement store. Studs are evenly spaced behind walls. In most houses, studs are spaced 16" apart. Studs are usually 1. 5" in width. When you attach an object to a stud, you need to attach it in the middle of the stud, which is 3/4" from the edge. Try finding an electrical outlet on the wall. One side of the electrical outlet will be nailed to the stud. To locate which side, you can do the knock test. Using the heel of your hand, gently knock the wall on each side of the outlet. The side without the stud will sound hollow, while the side with the stud won’t. After you have determined the side with the stud, measure 3/4" from the side of the electrical outlet. This will be the center of the stud. Using a tape measure, you can mark the studs along the wall every 16". Another way to locate the studs is to look at the baseboards. Baseboards are nailed to studs, so if you find holes or indentations that have been painted over or caulked over, you can measure 16-24" inches from that point to locate additional studs.

Drill holes through the cleats and into the wall. Make sure each hole is drilled into the center of the studs you marked earlier. You can also nail the mounting boards to the studs instead of drilling holes. If your wall is brick, make sure that you drill into the brick, not the mortar. Mortar doesn’t have structural strength, so you want to avoid attaching the shelf to the mortar. [4] X Research source Use a hammer drill, concrete screws, and masonry drill bits. If you are drilling into softer brick, a carbide-tipped bit in a power drill can work. [5] X Research source Drilling into brick takes a lot of force and power, so make sure you have the holes exactly where you want them. When you have the holes, complete the task by screwing the cleats into position using the drilled holes. Be sure to turn your hammer drill to regular drill mode before doing this.

You can also nail the mantel to the mounting boards if you wish.

Apply wood putty to cover the screw heads, taking care to smooth the putty evenly with the surface of the mantel. Allow the putty to dry, and then paint to hide the holes completely.

If a mantel is 10" wide, the minimum distance from the top of the fireplace is usually 19". For an 8" wide mantel, the distance is 17", and for 6" it’s 15". After levelling the shelf, draw a line on the wall that corresponds to the edge of the mantel. Make a mark in the direct center of your fireplace. You want to make sure your mantel isn’t one-sided.

Measure the length of the cleat. Then, using that measurement, find the center line and mark that on the cleat. [7] X Research source You will line this mark up with the mark you made on the wall in Step 1. The top of the cleat needs to have an angled edge, not a straight edge. Take a saw and cut a 45 degree angle lengthwise along one edge of the cleat. This is what the mantel hangs on. Dry-fit the angled edge of the cleat into the mantel. Make sure they fit snugly together so that the mounting board will support the mantel. If you do not want to saw an angled edge, you can use a flat edged cleat. Make sure your cleat is wide enough for the mantel to be fastened to the cleat with screws.

If you do not want to measure the two pieces together, measure the length of the mantel and the length of the cleat. Add those two measurements together to determine how far to draw the second line beneath the first line.

In most houses, studs are spaced 16" apart. Studs are usually 1. 5" in width. When you attach the shelf to the stud, you need to drill or nail into the middle of the stud, which is 3/4" from the edge. If you don’t have a stud finder, try finding an electrical outlet on the wall. One side of the electrical outlet will be nailed to the stud. To locate which side the stud is on, use the heel of your hand and gently knock the wall on each side of the outlet. The side without the stud will sound hollow, while the side with the stud won’t. After you have determined the side with the stud, measure 3/4" from the side of the electrical outlet. This will be the center of the stud. Using a tape measure, you can mark the studs along the wall every 16".

If you are attaching the shelf to brick, try for around 5 screws. [9] X Research source If you are attaching the shelf to dry wall, drill or nail into the studs. Drill holes into the wood before you fasten it to the wall. This helps the wood not to split. [10] X Research source

If you are using a flat cleat, fit the shelf over the mounting board. Then, fasten the shelf to the cleat along the back edge near the wall. You can either attach the mantel to the cleat using nails or screws. When attaching the mantel, you want to try to drill or nail in the center of the side of the cleat.