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'll see an icon that looks like a landscape. After tapping on it, you’re able to choose an individual background from your camera roll, and ultimately, upload it to the app.