As Pat mentioned, clear vinyl shower curtain works great. Then get the wet erase markers from an office supply store. Personally, I am not at all crazy about the technique. I prefer to draw out my block design on paper then use tracing paper to audition motifs. If I come up with something I really like it goes into my portfolio of quilting ideas and I can do it from the comfort of my sofa without having the quilt there. Finally I am not risking the quilt at all by accidentally marking it.