Sand the surface of the existing stair treads with medium-grit sandpaper. Sanding the treads will remove any finish and offer a greater bond between the existing treads and the false treads.
Vacuum the stairs to remove any dust and debris. Wipe down the treads with a damp cloth to further remove any excess dust and dirt.
Apply wood glue to the back of a false stair tread. Apply the glue around the edges of the false tread and along the middle in a wavy line.
Position the false tread in place over the existing tread. Press on the top of the false tread to create a bond between the treads and the glue.
Hammer finishing nails along the edges and the back of the false stair tread to drive them into the existing stair tread.