How to increase stash size in DMZ in Modern Warfare 4

One problem that everyone is going to encounter in Call of Duty: Modern Warfare 4's DMZ mode is how to increase your stash size. It's a common recurrence in pretty much every extraction shooter, because your stash always has limited space. I'm pleased to report that you can upgrade your stash and increase the number of items you can store in DMZ, but figuring out how is a task in itself.