Cover your workspace with newspaper to limit the mess.
Measure and trim your paper/newspaper to fit the outsides of your boxes and their lids. Use spray adhesive to attach the paper to the outsides of your boxes.
Water down your paint to make it thinner and more easily spreadable, or just use watercolor paint. Paint the boxes thoroughly and allow them to dry completely.
Mix a solution of one half glue and one half water to make a sealant. Completely cover your boxes with the glue mixture and again allow to dry.