Wipe the mantle with a tack cloth to remove any dust.
Cut a 15-inch-square of lint-free fabric. Lay it on a flat surface. Fold the side edges so they meet in the center. Fold the top and bottom edges to meet in the center. This keeps the loose threads on the cut edges from snagging on the wood and creating lint.
Apply a generous amount of tung oil to the cloth and wipe it on the mantle wood starting on the underside. Wipe the oil on the entire underside of the mantle. Apply more oil to the cloth when it no longer slides smoothly over the surface or when the oil no longer covers the wood completely as you wipe. If the oil puddles on the wood, wipe it away quickly.
Apply the oil to the sides of the mantle and then to the top. Finish the front last. This will keep you from inadvertantly touching the oiled surfaces as you work.
Allow the oil to penetrate for 30 minutes and then wipe the surface with a clean lint-free cloth. Fold this cloth as you folded the first one. Wipe until the surfaces are dry and have a low sheen.
Apply two more coats of oil in the same way allowing the oil to harden for 24 hours between coats.