Mantle installation Mantle installation Step 1. A blank slate, the stone was installed around a plywood mock up of our mantle, leaving just enough room to get the mantle inside the stone. 190395395 Step 2. Determine a line to mount the primary cleat on. This will provide structure to mount the cornice box on. 190395396 Step 3. Installing the mounting cleats, in this case a 2 x 4 with plywood bottom. 190395397 Step 4. The cornice box then is installed on the cleats, screwed on from the inside. 190395398 Step 5. A top cleat is installed to support the mantle shelf. 190395399 Step 6. The mantle shelf is installed to the cornice box and cleat using brads and construction andhesive. 190395400 Step 7. Apply a decorative trim around the shelf and cornice box. 190395401 Step 8. Stand back and admire! 190395402