While mantel shelves provide a big visual impact, their installation is very low impact. The first piece of information you need before you can begin laying out the pieces and parts for a mantel (or "mantle" as it's sometimes misspelled) is to know exactly where you want the finished shelf to go: left, right, up and down—and what its proportions are. If it's working as a mantel, you'll want the shelf and corbels to extend proportionally past the left and right sides of your hearth. As a rule of thumb, 6 inches between the edge of the hearth and the inside edge of the corbel is a good place to start. I'd also come about 8 to 12 inches from the top of the hearth to the bottom of the mantel/shelf. This creates a proportional reveal. For more, or to get started building and installing your own mantle shelf, watch this DIY video tutorial.