It's a standard Citadel Fine Detail brush I used, I just used some pencil lines to position where I wanted the lines to go. I work as a freelance illustrator, so I have some experience doing straight lines.
Another trick you can do is get artist tape (hardly any adhesive on it what-so-ever)
to use as a bleed-off for making straight lines. If you're worried about getting sticky stuff on your model, you can even stick the tape on a clean surface before putting it on your model to remove some of the adhesive.
I had to pencil in most of the text and the frame of the banner emblem before I started painting, too...it was not fun.