But before fastening front/back we need a solid spine. For this you need a piece of (80g) paper and some fabric (old textile shopping bag, whatever), about 2 times as wide as the back of your book (note: you can also use only the fabric without the paper if you prefer a fabric-looking spine). Glue the two together, apply some more glue to the bookback (only the back, be careful not to drip on the sides of your book) and apply the Spine you just made (paper/fabric should still be glue-wet)