Apply downward pressure on the cracked stair tread with your hand to open up the crack enough so that you can squeeze wood glue into it.
Apply wood glue inside the cracked area of the stair tread and release the downward pressure on the tread. Push the crack together firmly with your hands.
Clamp the stair tread using a wood clamp. Wipe off any excess glue that oozes out from the stair tread with a damp rag. Allow the glue to dry for 24 hours and remove the clamp.
Hammer one finish nail on each side of the crack in the stair tread in the ends of the repaired stair tread. Drive the nails into the tread at a slight angle to provide a more secure grip. The nails will reinforce the stair tread and help prevent further cracking.
Center a nail set on the head of each finish nail and tap the top of it with a hammer to set each nail approximately 1/16-inch below the surface of the wood.
Fill each nail hole with wood putty using a putty knife. Allow the wood putty to dry for 24 hours and sand it flush with the surface of the wood using a fine-grit sanding block. Wipe off the sanding dust with a tack cloth.
Touch up the filled nail holes with a wood finish touch-up and a small paintbrush. Allow the touch-up to dry for 24 hours before heavy use.