Measure your mantle. Use a tape measure to get the length of the face of the mantle and the two sides. Write down the dimensions. You will use them to cut the molding for the mantle.
Cut your molding according to the measurements. Use your miter saw to cut the molding pieces for the front and sides.
Attach the front molding piece to the mantle by putting a little wood glue on the back of it and hammering it on the mantle front with wood nails. Next, attach the two side pieces using the same technique.
Sand the surface of your mantle and molding for a smooth feel with a fine to medium grid sandpaper. Wipe away any dust particles with a cloth.
Prepare to prime the mantle. Place painters' tape around the wall area where your mantle is joined. Use a paintbrush to add a coat of primer to your mantle and molding. Allow it to dry completely.
Use a paintbrush to paint the top coat of paint on the mantle and molding. Allow it to dry. Add a second coat of paint to the mantle and allow it to dry completely as well.
Place decorative items on the mantle as desired.