I have finished the card holder book. When I left off (here), I had painted some large areas of blue in the two of the corners. In order to help these blend into the white areas, I used a brayer to put white acrylic paint on one side of a plastic doily. I set the doily down on the cover and used a different brayer (with no paint on it) to smush the painted side of the doily to the cover. I did this in both corners.
I still felt it needed more, so I took a rubber stamp, applied white
paint to it (I used a brush, but could have used the brayer with paint
on it). I stamped it a couple of times on the cover, trying to roughly
align the pattern as I went.
Closeup. You can see I added some details to the birds (wings and eyes).
I made a box for the cards from scratch using a cereal box. This was a real pain, so I highly recommend finding a pasta, rice or similar, box that will work. It would be a good idea to pick your box first, then pick the book you are going to use, as they need to be a similar depth.
In the mean time, I did paste down decorative paper on the interior of the book. For the box, I used a paper decorated with white paint using a bubble technique (from my monthly art group). I decorated it with the same bird stencil as the cover, but layered a checked stencil under it as well.
Here it is compete.