You attach this to the bottom of your application! you do this by editing the canvas size, and adding 554 pixels to the bottom of your application. Then, you copy this image over to the application.
You only need to use an item ONCE to unlock it. After that this is permanently a part of your app! If you already unlocked *Stacking* items, aka being able to write "accessory voucher x3" by paying a scroll or voucher, then you automatically have this part of your app unlocked.