The simplest and easiest homemade adhesive you can make is a paste from flour and water. Here's a quick version that does not require any cooking. It works because the water hydrates the molecules in the flour, making them sticky.
The simplest glue is made from a paste of flour and water. You can also make cornstarch paste or milk glue. All are easy, non-toxic, and great for making paper crafts including paper-mâché projects.