Houses of a certain age or with un-improved basements feature open stairs containing only stringers and treads. The fascia that covers their risers may be missing. In addition to making your stairs safer, covering the risers may be required by your local building code if you plan improvements to your cellar.
Measure each riser from the top of the tread below to the bottom of the tread above. Tread heights might vary because of adjustments on stringers, the 2-by-10 bases and tread thickness, but are generally near 7.5 inches. Record the height of each riser on a sketch of the staircase.
Cut 1-by-8 boards to fit the width of the staircase. Depending on your steps and preference, this may be the distance between the outer stringers or from wall-to-wall on enclosed stairs.
Remove enough width on each board to fit a specific tread and label it with the number of the step, beginning with number 1 at the bottom. If you use a table saw, cut the edges of the riser at a slight angle toward the center of the back where you have marked the riser number. Otherwise, sand the edges to achieve slants back from the front of the fascia.
Pull the treads from the stairs, starting from the top, labeling them on the back as you go so each tread's number matches the riser fascia that will sit under its front. Stack the treads and riser fascia at the bottom of the stairs in order with the top pair, that will be the last used, on the bottom.
Set the first fascia against the riser and nail it to the stringer -- use two nails on each stringer. Apply a light line of construction adhesive along the top of the fascia and set the tread on top of it. Wiggle the tread back against the stringer and nail or screw it back in. Repeat setting riser fascia and treads up the stairway, checking each riser to ensure that it is perpendicular and that the treads lies flat as you go.