A number of recipes for making homemade chalk paint are listed on Salvaged Inspirations. These recipes involve mixing paint with other materials such as baking soda, grout and calcium carbonate.
One of the simplest recipes for chalk paint is paint mixed with calcium carbonate, which Salvaged Inspirations claims yields the best results. To make this type of chalk paint, mix one part calcium carbonate with two parts paint. Calcium carbonate is chalk, so it creates the most chalk-like texture for the paint. However, the recipe calls for calcium carbonate in powdered form; this can be ordered online but may be difficult to find in stores.
Plaster of Paris is an easy-to-find substance that can be mixed with latex paint to create chalk paint. To make chalk paint with this method, mix one part plaster of Paris with one part water. When a smooth paste is formed, add three parts latex paint. After it is completely mixed, the paint can be used immediately. Both Salvaged Inspirations and Lowe's recommend this recipe.
Salvaged Inspirations also lists recipes for making chalk paint with baking soda and unsanded grout, both of which produce a more textured paint coating and may be a good choice if an aged look is desired on the painted surface.