Measure the distance from one side of one step to the other. Cut off the end of the stair nose molding with a miter saw to ensure a square cut end from which to measure.
Place the end of the measuring tape on the square-cut end of the mold and mark it according to the width of the step. Cut the mold with the miter.
Make a mark 1 inch from each of the ends of the mold. Measure from one end of the piece of the stair nose mold and make marks every 6 inches. Since most stair nose molding hangs over the front edge of the step, be certain the marks are far enough from the front edge of the mold that the nails will secure it to the step.
Drill pilot holes on the marks with a 1/16-inch drill bit. Drill from the top side of the mold so any splinters that occur due to drilling are on the bottom side of the mold.
Turn the mold over and sand the drilled areas until the splinters are removed. Turn the mold back over and set it in place on the step to check its fit. Adjust it if necessary.
Secure the mold to the step with the finish nails. Drive each nail in just short of the surface of the mold. Drive the nails to just below the surface of the mold.