Place drop cloths on the ground below the area where you'll be working. The drop cloths will help protect your flooring.
Apply primer to the walls. Cover the oil-based paint with the primer. The primer will help the latex paint stick properly to the oil-based painted walls. Use a paintbrush and paint roller to apply the primer. Let the primer dry completely.
Mix the latex paint with a paint stick. Pour the latex paint into the paint tray. Roll the paint roller into the paint and apply to the wall. Roll the paint onto the wall in a “W” motion. This will help you avoid paint streaks on the wall and get good coverage. Apply a second coat of paint as needed. Each coat of paint should dry before applying another coat.