Wash the fireplace enclosure with warm sudsy water, and let it dry completely.
Prime and paint the brick fireplace in a color suitable to the rest of the room. For instance, white is a popular color to clean up a red brick fireplace. If you want to step it up a notch, paint a vibrant shade of cherry, cerulean or lavender depending on existing decor. Or, if the room is already rich in color and pattern, choose a neutral hue like taupe, gray or cream.
Create a shelf for the mantel with the length of wood. Cut it to the length of the mantel. Whether you choose to use a 1-inch by 6-inch or a reclaimed piece of wood from an old fence depends on the look you wish to achieve.
Apply a thin layer of construction adhesive to the edge of the shelf where it will meet the bricks. Affix the wooden shelf to the brick fireplace using the L-brackets and masonry screws.
Frame the shelf with decorative molding to finish the mantel. The mantel will hide the L-brackets and provide a clean finish to the fireplace. Either choose white molding or dark wood molding for a modern look.