You can customize most of the designs - which means for example - lets say the shirt advertised is green, but you'd prefer a black or red one - you click on "customize" - over to the right under the enlarged photo - and it will give you options for shirt style or color and so on. After that is done - you can add text - like maybe your own name or a favorite Quote. Have fun.
To make your own goodies - set up a user account - its free to do. Decide what you want to make - what image you want on it - and then get started - there is a "create" button at the top of the pages. You can decide to publish the item for sale or keep it private only for you to buy when you want it. If you publish an item, don't forget, if you add Fornits to your product tags, it can be added to the store here - which will net both you and the Forni a bit of fed.
Zazzle is pretty good about clearly walking you through the process, and they seem to be adding new features all the time.