Measure the wall for the length of molding needed.
Cut the 3/4 round molding to fit the length needed. If the molding will continue around a corner, make a mitered cut at a 45-degree angle at each end of the molding, so that the molding on the adjacent wall fits flush together at the corner. The most efficient method for making mitered cuts is using a miter saw or table saw with a miter gauge.
Place the molding against the baseboard to assess the fit. Make any adjustments to the cut necessary for the molding to fit flat against the baseboard.
Nail the molding into the baseboard using a pneumatic nailer and 2-inch wire nails. Wire nails create less wood splitting and leave only a tiny hole to patch.
Cut and install molding on all the remaining walls using the same method.
Fill the wire nail holes with a bit of wood putty, applied with a small plastic putty knife.
Sand the patched holes by hand with fine-grit sandpaper and wipe away the sanding dust with a microfiber cloth.
Touch up the patched holes with the paint or stain used on the molding.