Whatever you decide to use, test on scraps first to see if the marks come out/off.
Avoid ball point pen - someone used that - it was a very bad idea. Many people also say to avoid using yellow, blue, or pink pencil/chalk type markers
One can get pale gray General Chalk Pencils - I've had good luck with the white ones rubbing off other fabrics.
I've used a lead/graphite pencil - very lightly so you can barely see the marks. I made heavy/dark lines ones once on a quilt top - thinking that the lines would rub off or be easily erased - Not!!!
Some people have had good luck using the Crayola washable markers. I have bought some, but not used them yet.