Smooth Finishes
High-quality plywood with a smooth surface works well. You will need to caulk all gaps after you install each riser with nails or screws. It's possible to paint most of the risers' surfaces before putting them in place. However, touching up all painted areas after risers are in place and blending in caulking smoothly will ensure a professional looking job.
Sizable Thickness
Thicker plywood works best. Use 3/4-inch plywood, for example, for a sturdy surface on the back of every step. Thin plywood sheeting that is only 1/4-inch thick may warp over the years.
Good Painting Techniques
High quality paint is critical. Don't necessarily use high gloss paint, because it will tend to show up imperfections. Use semi-gloss, however, because you can clean it more easily to remove any scuff marks from stair traffic. Apply the paint to the plywood with a sponge or roller, because you don't want any brush marks showing up as you face the stairs.
One-Piece Cutting
Large sections of wood prevent the need for seams. You don't want any seams on risers, because the plywood will eventually shift and create a bulge, destroying the look of the surface. Even if you have to waste some plywood, cut out the risers from one piece of material. Risers are so prominent on any set of stairs that their entire surfaces will show up the moment you start to climb the stairs.
Future Ideas
In the future, you can add tile to the painted plywood surfaces. If the treads of the stairs are solid wood, for example, you might want to add colorful tile work to the front of risers in the future. It's easy to add glue and grout to the plywood fronts; however, you will need to sand the painted plywood to prepare the surfaces to hold the tiles.