Pretty easy to get, each shield you find in the main game unlocks either a character or another unlockable. Getting all the pup snacks and increasing it to the maximum for each level and each mini game does the rest. You can replay any level or mini game to find any you may have missed, you don't need to.get every pup snack each time, the ones you missed with be a different colour so you can ig ore the others to make it a quicker task. Once you get every shield and every pup snack you're all done and it will pop.