If you plan to use this mocking library as a tool in your test-driven development workflow, you've come to the right place—it's what Mocktail was designed for!
There are several flavors of test-driven development, but the relevant distinction for figuring out which direction you want to take Mocktail lies before you:
Use Mocktail to create fake instances of Ruby classes you own.
Use Mocktail to fake out class and module methods of types you own.
Use Mocktail to fake out third-party code you can't readily change yourself.