Just any crayons do not hold in cotton fabric... and the fabric crayons say at least 35% poly to hold the color.
There are some crayons that do hold good on reg cotton fabric and I cant find them right now. They have a higher color pigmant. You do heat set all. The fabric crayons you draw on paper and iron on so if doing words draw the picture and trace on a window and color the back side. The it will be the right direction. Good luck... oh otherwise iron the muslin to freezer paper to draw and color. Also draw with a permanant pen then color inside the lines like a color book...