We will make a hard crease at the spine edge of every signature, using a bone folder. These are starting to turn up at craft stores, and at very reasonable prices. Teflon folders are also being made and work very well. However if you don't have or can't find one, anything of similar shape, ideally of plastic or wood, will work (a plastic letter opener, for instance, usually works well). Metal will work too, but sometimes marks the paper. It would be a good idea to do a test run with whatever you use, if other than bone or teflon.