Cards are a vital resource in Lost in Random. As such, you will need someplace to store them, and childhood should have taught you that the best place to store cards is a binder. This is no different in Lost in Random.
To get the binder, you need to head to Two-Town. There you will find Mannie Dex who will give you the binder as a gift for visiting his store for the first time. This allows you to expand your card collection.
Related: How to use cards in Lost in Random
With the binder in hand, you can now modify and customize your deck during combat. You should have 15 cards at this point, but you won't be able to customize your deck until you have at least 16 cards. Since your deck must have 15 cards in it, it will default to whatever cards you have until you exceed that number.
This allows you to experiment with new card combinations as you collect more cards. With different types of cards and multiple cards in all availalbe categories, there are plenty of combinations to try out. In your binder, you can view all of your cards, their artwork, and read their descriptions. As an added bonus, you can also relive your childhood memories of toting around a giant binder full of cards.
For more information on Lost in Random, check out How to get the Menace Achievement in Lost in Random and How to get the Go Fetch Achievement in Lost in Random on Pro Game Guides.