I made one for my baby showers when I was pregnant and already had the top put together for people to sign. I saw an article in a quilting magazine a while ago that suggested you iron the strips/pieces to freezer paper for them to sign. The freezer paper stabilizes the fabric. When you are ready to piece it together all you have to do is peel the freezer paper off. This article suggested that you clearly mark the stitching line and give instructions to "stay in the lines"!!! The same article also suggested a "practice" piece for people to sign. Good luck!
Rose Hall