First, purchase a UPC bar code in the project quoter.
If you're doing design off-line, plan ahead for where your bar code will go. It doesn't need a white box, just an area with no critical text or images. (Our barcode is .5" tall by 1.25" wide.)
After you upload your completed PDF files (or create your artwork using our Online Designer), you will be taken to a screen that allows you to choose the location for your UPC bar code in your artwork as seen here: