Are you trying to put a design on them or hem the edges. If you are trying to put a design on them try doing what you'd do if you were embroidering them. First of all you can use a hoop that doesn't attach to the machine (if using a regular machine. Use stablizier on the bottom and water soluable stablizier on the top. The top stablizier keeps the stitches from sinking into the fabric. If you are trying to hem with a zigzag you may have too tight of a stitch.