I’m pretty sure a lot of people have done this before! I used small paintbrushes on this (this was two years ago, and there weren’t any pens yet from where I come from).
A little something I did for UN day back in 2008 (along with 9 other flags which were easier to paint). Guess where I’m from? :3
It’s quite messy, though it’s relatively easy to do. I used a small paintbrush for the sun and the stars.