Start by washing the mason jars with mild soap to clean - let dry.
Add some Mod Podge to a small container (a piece of Tupperware works great).
Put a couple of drops of food coloring or paint into the Mod Podge. Stir.
Apply the Mod Podge in one of two ways: 1) use a soft bristled brush, spouncers, or foam brush to apply to the outsides of the jars; 2) swirl the Mod Podge around the inside of the jars and drain.
Regardless of the process you choose to apply the Mod Podge, let dry for 24 hours before using the jars.
Video
Notes
If you find that you're having difficulty drying the Mod Podge, you can put the jars in the oven at the lowest temperature for several hours. Just be sure to never leave the jars unattended and check them every 20 - 30 minutes.