Place baseboard on top of a flat surface. Make sure you protect the area around for paint stains
If your baseboards are smooth all over then use a smooth roller. If your baseboards have grooves then use a roller for rough surfaces
User roller to paint the baseboard. Paint atleast twice and always finish up with on roll over the whole baseboard to avoid droplets to form.
Make sure baseboards dries before nailing the baseboards to the wall.