Clear out the expansion joint, removing all materials from the joint before installing the wooden joint sheet. Cut any caulking or joint sealer running the length of the joint into small 4-inch-long sections using a utility knife. Strip these sections from the joint with the edge of a putty knife to reveal the filler material inside the joint. Remove any filler material inside the joint with an old flat-head screwdriver. Remove any remaining dirt or debris from within the joint with a nylon brush.
Clean the cleared expansion joint by washing the interior of the joint as well as the concrete surrounding it with a mild liquid soap and warm water. Use a scrub brush to remove all dirt and any stains from the area. Use a concrete degreaser to remove any oil stains present, as the oil may interfere with the new joint filler materials. Rinse the concrete clear of cleanser residue with a garden hose, then wait for the concrete to dry before proceeding.
Use a tape measure to measure the width, depth and length of the expansion joint. Purchase a piece of wood fiberboard of the same thickness, long enough to fill the joint while being about 3 inches higher than the depth of the joint.
Slide the fiberboard into the joint. Mark the edge of the concrete along the side of the fiberboard with a piece of chalk. Pull the board from the joint and make a second line, 1/4 inch lower and parallel to the first. Cut the board along the new marked line using a table saw with a plywood blade.
Place the board back into the expansion joint. The wood helps to cushion the concrete slab as it moves against an adjacent concrete surface. The 1/4-inch space along the top leaves room for a sealant.
Place two strips of masking tape along the edges of the joint, one running down each side.
Fill the space above the fiberboard with joint sealing compound. Pack the compound in tightly over the board using the putty knife until it's at the same level as the concrete surface. Scrape the compound smooth with the blade edge then strip away the masking tape to leave a well-formed uniform line of sealer. Allow the sealer to set for 48 hours before using the surface.