I made the glue from 1 part corn starch and 10 part water by weight, microwaved until boiling.

I found that serrated knives cut cardboard best, but they need to be re-sharpened often. The cardboard absorbs a lot of glue so it need to be applied multiple times before sticking together.

I used paper to secure the joints between the cardboard pieces and to wrap the outside before painting. The glue doesn't really stick to acrylic paint, so it should be the final part, and you can't add anything else afterward. If I do it again I would leave to table bare, or maybe use watercolor