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If one or both sides of your stairs are exposed (not against a wall), you'll need to apply return trim for a proper finish. The return trim typically comes pre-mitered on one end. However, if your walls are not perfectly square, you may need to measure and cut the proper angle using an angle gauge and a miter saw. Once the trim is properly fitted and dry-fitted in place, attach it with a small amount of polyurethane construction adhesive and secure it with three finish nails. Finally, use matching wood putty to hide any visible nail holes for a seamless appearance.