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’re 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 a little icon that looks like a t-shirt. Tap on it. Here you’ll find all the stickers from the category "popular." Change the category by tapping on the scissors icon in the upper right corner. Click on the plus.
Now, you’re camera roll is going to open up where you can choose an image from. So choose any one image and cut it to your liking!
The creation of stickers is pretty similar to the creation of own items in the categories. Wondering how you can create/upload your own items? Find out here