Cut a piece of cardboard the size of your intended book. Here, I have reinforced an undesirable crease in my cardboard with "Yes" Paste, a sliver of cardboard and black cardstock.
Here, I have lined the inside with black cardstock and the out side with a sheet of beige denim paper.
I continue the process by collaging over the spine with a slightly darker flax and cattail handmade paper and with grass paper on top of that. I let it dry overnight, but mostly because it had gotten late.
As it has again tonight.