A place where nobody suspects.
Step 1: Take the Back Off
You will have to unscrew or pry off depending on what kind of watch you have.
Step 2: Take the Insides Out
You might need to break some things in there, so grab some tools necessary for doing so.
Step 3: Put a Different Back on It.
I chose to tape a piece of paper onto it since nobody is going to be able to see it, but you can use whatever you want.
Step 4: Putting Things in It.
Simply lift up your cover, put something in, and put the cover back.