Tear off several small (about 8-12”) pieces of 1.5” (or wider) painter’s tape.
Align the tape along the baseboard with about 1/4”-1/2” of tape folded up along the edge of the baseboard. This amount will vary a bit depending on your carpet/baseboard situation.
With a plastic spackle knife, gently push that excess edge down slightly underneath the baseboard.
Tape edge will be between the board and carpet. Continue until the entire length is pushed down between the baseboards and carpet.
Tape the entire section of your baseboards this way.
Load up your brush with primer and push it along the top edge of the baseboard, working in small sections. No need to tape off that top edge if you are careful!
Continue painting with primer all the way down to the taped edge. There will technically be a tiny edge of baseboard that will remain unpainted when you later pull the tape away, but the carpet will completely cover that.
Apply another coat of primer (optional) and two coats of Benjamin Moore Advance. The Advance paint requires a 16 hour wait between coats, so plan accordingly.
Before your last coat has completely dried (must be dry to the touch to prevent smudging!), push the tape away from the baseboards and down towards the carpet to break the seal formed by the paint.
Tip: If your layers of paint are too thick, use a utility knife (or a metal spackle knife) to gently score the paint and break the seal.
Pull away the piece of tape. This can be super easy or a bit of a challenge depending on the day!
If you have any little ridges left where the tape was, I like to grab a piece of fine sandpaper (like around 220 grit) and buff away any imperfections. And you're all done! Enjoy your freshly painted baseboards!