Wash the baseboards with a rag and soapy water to remove scuffs and dust. Dry the baseboards with a clean, dry rag.
Examine the baseboards for nicks. Use a small amount of putty to fill in unsightly damage and create a smooth painting surface.
Apply painter's tape on the wall above the trim. The edge of the tape should sit flush against the edge of the baseboard. Stick tape to the wall slowly so it does not wrinkle or crimp.
Line a sheet of newspaper against the wall where you plan to begin painting. Press the edge of the paper as close to the bottom of the baseboard as possible, or slightly underneath if you can.
Dip the paintbrush in the paint and drag the brush against the edge of the paint can to remove excess paint. This will reduce the possibility of drip lines as you paint the trim.
Start at the end of the baseboard and apply paint from right to left. Use the angled side of the brush to apply paint to the top edge. Some paint will brush against the tape, and this is fine.
Continue painting the trim, moving the newspaper to each new section as you paint.