Apply wood stain to the new stair treads using a paintbrush. Brush it on in the same direction as the wood grain, covering the top and sides of the tread.
Wipe off any excess stain using a lint-free cloth to prevent a blotchy look.
Allow the stain to dry completely without disruption. Your stain may look dull and lighter than expected after it dries, but that will change after you seal it.
Use a paintbrush to apply a coat of polyurethane over your wood stain. Apply the sealer in the same direction as the wood's grain. Allow it to dry.
Sand the polyurethane lightly, then add another coat. Use at least three coats of polyurethane on your new stair treads to help protect them from wear and tear.