Joining the paperblock and cover is fairly easy but is often the most stressful and the first couple of times you do it there might be some ripples in your inside cover, this can be fixed by waiting for it to dry and then sticking the first page to the inside cover.
Note: I forgot to do a prior step so the photos are misleading. The step missed was trimming the mull and ribbon so it becomes a straight line, doing this is a good idea if you're after a neat book. I realised just before gluing and had to redo the position and gluing preparation.
First you need to get the paper block into the perfect position you want it glued from. It will look finished, don't get excited yet!
Open the cover without disturbing the position of the paper block. Get some newspaper or scrap paper and slide it underneath the front page. This is important because the glue needs to get right to the edge, and if you dont guard the edges, glue goes everywhere into the book.
Practice shutting the cover as if there is glue on the page. Usually I have my right hand supporting the paper block making sure it is stable but I was taking the photo.
Ok the gluing, you need your wallpaper glue and your other bookbinding glue that you did the spine with. I don't have photos but these are the stages of where you lay glue. First, underneath the ribbon/tape put pva glue.
Then ontop and between the tape
Push the mull down into the glue, and the another layer on top of the jaconet
Do the same again if you have two layers of mull. At this stage the area covered by the mull should be glued with the binding glue, then you grab your wallpaper glue to cover the rest of the page. Don't clean your brush before dipping it in, the mix of glues is helpful, even drag excess glue from the left over onto the rest of the page.
The final step is to push the edge of the cover down so there is glue on it, then basically close the book. Just close it, apply a bit of pressure by squeezing it with your hands. In my experience it is not helpful to rub things down, just let the glue do its job. Before the glue has set but not immediately after closing the cover, open the book to check for problems/air bubbles/creases/extras glue additions/glue overflows Thick papers tend to have less of these problems. Don't over do it with the glue, be confident in your glue's ability to do its job.