1. I don't know if this is right or not, but this is how I figured out how to do it... I used window/door trim caulking and did the corners and nail holes in the trim.
2. I also did the top and bottom to fill in the small gaps, no wall or ceiling is perfect after 10 years, or in some cases, 10 seconds, after they're installed
3. The base trim got the same treatment, the corners and where it meets doorways gets filled in too.
I think it gave the room a clean look.