Measure the length of each stair step. Each stair step may have a slightly different length and should be measured separately.
Rip each false tread on a table saw to match the length of each stair step. Cut the ends of the false tread that will abut the skirt board, if the stairs have an open side.
Sand the cut ends to remove any frays left from sawing.
Wash the old stairs. Rough-sand the old finish from the tops of the stair steps to ensure adequate adhesion of the new treads.
Apply beads of caulking adhesive around the perimeters of the undersides of the false treads. Apply zigzagging beads down the centers of each tread, then stick them to the tops of the steps.
Nail the false treads in place. Drive 2-inch long finish nails at each rear corner of each tread and in the rear center of each tread.