I too took on a signature quilt without planning a block in advance. My Grandmother's 80th birthday was a surprise party where many friends and family would be. I picked out pastel colors and cut 4 and 1/2 squares for signatures. I still haven't completed the blocks. I just wasn't happy with the limitations I put on myself without proper planning.
One thing that is very beneficial is to iron freezer paper to the back side of fabric to stabilize fabric for better control of signing. It works great!!!!!! Good luck