Generally speaking, you don't. Most of the canvas I buy at the craft store is pre-primed, so I don't have to worry. Even if you buy canvas for DIY wall art and it's not primed, you'll be fine just to start painting. Having said that, you might have to add a few more coats of acrylic paint just because it will soak in. The paint also won't flow quite as well across the top of the canvas. What you can do if you want to prime your canvas is paint it white, or add a coat of Mod Podge over the canvas. Then you can paint on top with ease.