Apply painter's tape along the baseboard, ensuring that 90% of the tape's surface is on the carpet. Use a strong adhesive glue tape for a secure hold, avoiding light masking tape that can easily come off. Leave a quarter-inch overlap on the baseboard to ensure the tape stays in place.
Now take your putty knife and starting at one corner of the room, began to push the tape down so that it is wedged between the baseboard and the carpet. Next, run your hand over the painter's tape to make sure that it is stuck to the carpet. Now, you're ready to prime.
Apply primer to baseboard with angled brush using short strokes. Work on 2-3 ft sections, cover all unpainted areas. Wait 30 minutes to 3 hours for primer to dry. To avoid drips, pour little paint and cover no more than 1 inch of brush with primer. Wipe off excess primer from brush.
Once the primer has dried and is ready to go, take your 120-grit sandpaper and lightly brush it over the trim. Be sure to start in the same corner in which you started the primer application. This helps to get rid of any paint drips or brush strokes. If there is any sanding dust, use your damp sponge to wipe it away.
Start painting the baseboard from the same corner with a new or cleaned angled brush, using short strokes. Allow the first coat to dry for the recommended time, which can range from 30 minutes to 3 hours (check the paint can for specific drying time). Once dry, apply a second coat, repeating the process until the trim has 2-3 coats of paint on all walls.
Wait for the paint to fully dry, then unveil your masterpiece by delicately peeling off the carpenter's tape from every baseboard.