The other 2 solutions are correct in telling the setup. I would add setting the draw timer to none, so you do not have to restart another round.
The paintbrush is the tool you want to use.
However, if you press down with the stylus (paintbrush), you create a wider brushstroke and use more paint (just like a real paintbrush). The risk you take is perhaps scratching the pad slightly so do not over press. Lightly touching the pad with the stylus gives a thin and lighter brush stroke and uses less ink.
Doing it this way you will run out of ink in less that 4 minutes. (I used a 2 minute timer, and before the second round ended I ran out of ink). The pad is resilient and I have no scratches.
I initially thought this would be a long boring achievement, but it ended up being a very quick one. Have a little fun with the paint brush at the same time.