Basically you create an account, then upload a piece of art to "My Fab Gallery" (it's one of the tabs)
Must be a JPEG, make sure you have like a 150 DPI and the like for the best print. You just kinda pick your size and adjust your picture how you want, decide if you want the image to wrap around the edges or have a solid border, if you want one canvas or multiple.
Before you get scared off by the price, they run deals through either Groupon and Living Social (I forget which) where you can get up to like 80% off lmao. They can be SUPER cheap if you just wait for the right offer ;D