Now that you have your signatures sewn, it's time for the next step.
Sand paper on a sanding block
(I've used a piece of sandpaper applied to a remnant of a board)
Liner paper (regular white paper)
Your sewn signature
First, you will need to line your boards. Using a PVA mix glue, glue paper liners onto each side of your boards. This paper can be just regular weight printer paper.
When dry you will sand off the excess.
Sanding off is much easier than cutting.
Now place the signature between the boards. Pull out the signatures until they stick out about 1/4 inch. Hold the whole thing with a weight.
You want to get a bit into those round edges, between the signatures.
Rub off excess glue with your thumb. Let dry almost 5 minutes, or until damp but no longer sticky.
Lift and pull the fore-edge of your text block, then roll your thumb over the top of the spine to start to round out the spine. You are trying to get the spine to curve in at the top and bottom (not the head and tail of the book). When you have worked over one side, flip the text block and work the other side. You can use a rubber hammer on larger text blocks. This small one won't show a lot of curve.
Soon: The naked part, or in other words, Decorating the Boards...