Measure the width of the top of the fireplace opening and add 6 inches to each side.
Lay the log on its side and cut it with a chainsaw to the dimensions taken in Step 1.
Stand the log up and cut through the center of it with a chainsaw, making a cut that is parallel to the length. Cut the log so that you have an 8-inch-wide section to work with.
Cut 2 inches off the back of the log to create a flat surface to hang against the wall.
Hammer a chisel gently between the bark and the wood to remove the bark from the surface of the log.
Run a block plane across exposed interior of the wood on the top and back of the log, smoothing and leveling out the surface.
Sand the wood with a heavy- to medium-grit sandpaper. Sand the sections again with a fine-grit sandpaper to create a smooth surface.
Apply a coat of polyurethane sealant to the entire surface of the log mantel with a paintbrush. Allow the sealant to dry for 12 hours and apply a second coat. Allow the second coat to dry for 24 hours.