Tape off the floor at the base of each side of the door jam to avoid getting paint on its surface using 2-inch masking tape.
Run a strip of 1-1/2-inch low-adhesive tape around the entire door jam, sticking it tightly to the wall surface with your fingers.
Lay down a drop cloth on the floor running through the doorway.
Remove the door from the jam by unscrewing the door-hinge screws with either a flat or hex-head screwdriver. Remove the door jam strike plate.
Pour 2-inches of paint into a hand-held paint container, and dip the paintbrush repeatedly until bristles are loaded with paint.
Paint the inside of the door jam frame first, beginning with the header and then down the sides of the jam. When finished with the inside area of the jam, run your paintbrush over the entire length of the inner jam, to blend all brushstrokes.
Paint the door jam trim on both sides of the door frame starting with the outside of the trim. Run your brush along this outside edge without allowing the bristles to touch the low-adhesive tape.
Paint the trim face starting at the bottom of one side of the door jam, working up over the top of the doorway and down the other door jam to the floor. Run your brush over the entire trim-face surface to even out all brushstrokes.
Remove all tape. Carefully rehang the door and reattach the strike plate once the paint is thoroughly dry.