Instructions
Join or create your own multiplayer room with custom missions. You do not need to complete all of the requisite custom missions consecutively or even in the same day, so feel free to take breaks between them.
Complete 50 custom missions. This is a very time-consuming task, but it does allow you to practice with the many weapons and their various accessories, so take the opportunity to practice and master a variety of different guns.
Save your game when prompted to do so. Once you complete the 50th custom mission, you receive a message asking you to save -- this indicates that the "Tea &Cupcakes" mission, also known as "M9," has been unlocked. This process also unlocks Maids as selectable enemies.