Fine Line Water Erase Pen
These are so handy -- when you need to mark your place or plan a pattern you can write directly on the material. Then rinse with water and the ink disappears! It's like magic, baby!
This pen is fine line and writes in a bright light blue which rinses right out. If you plan on ironing your finished piece, I recommend washing out before ironing as heat may “set” the markings.
You might try it out on scrap material first so you're totally comfortable.