What is the best way to paint it

Logically, you will save time if you paint the molding before installation.
Prepainting before cutting to size is more convenient, and looks best if spray
painted. With the large selection of aerosol paints available it may be a
trade off to getting the tools at a rental place. Dentil moldings always look
best if the well defined edges aren't blurred by paint gobs.
Considering the low prices these days on home compressor/paint guns you may
find one to be a good investment vs. rentals. HYH

Joe