The text block and cover of my handmade sketchbooks are constructed separately and glued together right at the end, before pressing. It's a very delicate and precise operation.
You need to make sure that the glue is applied evenly, that the end papers are glued on square both top and bottom on both covers, and that everything is kept is clean of excess glue. And you need to do it all quickly, and get the book into the press before the glue dries.