Yep, it is important to iron freezer paper to the back of the fabric to give it some stability for the coloring process. It helps to do the outlines over a light table using permanent fabric markers (NOT Sharpie!!!) or a pencil. There are also watercolor pencils available that can be smudged or watered down, but it is important that the fabric be prewashed to do so successfully.
I need a travel pillow for my upcoming trip and I think I'll make a crayon panel (no lines on my cheek) lol