Short answer: practice. I used a small brush, dipped it in the ABS paste, and carefully brought it to the exact spot I wanted. As the “paint” transfers to the helmet, I dragged the paint brush upwards when it starts to look like the drip is getting too large. The ABS paint sets very quick, so if it looked like it might start to run, I just gently blew on it and tilt the helmet back. The paint forms a skin very quickly so you can use that to keep things in place.
Oh, and luck.
I actually had no intention of doing this when I did but I was testing the consistency of the ABS paste and it was working on a piece of scrap, so I went for it.