Make a spacer bar or a jig so your stamps will be uniformly spaced on the risers. We stacked together some cut cardboard, making it about a 1/2 inch thick and the width of the stair treads. Make a strong pencil mark at the center for the stairs. Place the spacer bar on the tread to support the stamp and provide the center mark for the riser.
Use a mini foam roller to apply black paint to the stamp. Line the center of the stamp with the center mark on the spacer and press it into place, pushing evenly on all areas of the stamp.