Stick painter’s tape all around the outside of the window frame and on the inside of the glass to avoid paint messes.
Apply primer \with a paintbrush. Coat the whole window frame with primer. Allow the primer to dry for 24 hours. Rinse out the brush thoroughly so you can use it for the paint.
Paint the interior of the window frame, starting at the top.
Raise the window sash, if needed, and paint the casings on the windows. Allow them to dry.
Paint the trim of the window frame with broad, even brush strokes. Let the paint dry. Apply a second coat of paint, if needed for smooth, full coverage.