First you choose an outfit consisting of not more than 5 items. Next, you tap on “Edit & Post” in the upper right corner.
Now, you'll find yourself in the editing mode which enables you to move your items anywhere you want as well as enlarge and reduce them. In the upper right corner, you see an icon that looks like a paintbrush. Tap on this icon. Every time you click on it, a different background is going to appear.
Furthermore, you are able to swipe through a variety of backgrounds.