go to your kid's room and grab their washable markers (or you could go purchase some too). They are fun to use since you have a variety of colors to choose from. Plus kid's markers last for a long time. The price is reasonable as you get around 8 markers for about $4...which is about $.50 a marker! So far, I have never had a washable marker that will not wash out (even when the marker has been on the fabric for weeks). I have not tried the markers on delicate fabrics so I'm not sure if they will stain or not. If you are worried, test first.
Now grab your kid's marker.
Mark your lines and there you go! Just remember to wash your project once complete.