Hide small treats inside the pieces through their small holes, for your dog to nudge them all the way to the hole sections, before having to nudge the flap pieces away to further move their treat along and reveal it in the compartment where they can then grab it.

To make it a little easier for your dog when first starting out, you can just hide their treats under the orange piece, already in the hole areas for your dog to just have to move the orange pieces to reveal their snack.