The next important ingredient is powdered wallpaper sizing or adhesive. You can use papier mache for this craft instead, but I like the gel-like consistency of the adhesive; since it's less watery, it won't instantly dissolve the tissue paper you'll be using. It also dries super strong, is non-toxic, and is easy to clean up. One container will last you for years, and once you try it for papier mache, you'll never go back to flour and water. Trust.
For enough adhesive to make a whole tree, pour about a third of a cup of water into a container.
Happy haunting! xo