First...clean your piece really good! If using a chemical cleaner, wipe down with water after.
The most important thing to remember when painting fabric with chalk paint is...you want to "stain" the fabric, not "paint" it. This means water is your best friend!
Gather your supplies...a spray bottle filled with water (I'm using an old rinsed out 409 bottle right now), a mixing cup, your paint, a large paint brush, and a couple of small artist's brushes to get into the little creases.