Well, I think freezer paper is a bit heavy for paper piecing, that's why I now use tracing paper or just computer paper. However when I was using freezer paper, I put a used card on the stitching line and tore the paper against that. I'd guess that an old fashioned ruler with a sharp edge would work too. There are several companies that produce paper meant for paper piecing. In any case, keep a pair of tweezers handy to pick out the bits that don't tear away so easy.