It is best that you update your app to the latest version found here:

In the new app, it should auto populate your previous purchased, they will be found in the sticker selection, just scroll down until you see your stickers. So, take a photo, tap the sticker button and scroll. 

If you do not update the app and want to use the old one, please view below for help.

In order to use the stickers, you must go to the photo editor. Do this by taking a picture and then tapping the stickers button. On the sticker ribbon all of the stickers that are available to use will be there. You can slide the ribbon to view all the different options. 

See Video Example: