Instructions
Begin the game, and play until you are tasked with taking over the La Masion Rouge establishment. This will happen fairly early in the game.
Enter the La Masion Rouge through the back door and kill everyone who attacks you. Make your way into the eastern room, where you will find the owner.
Use your fists to punch the owner until she relents and gives you the business. Along with the business, you will automatically receive the brass knuckles. Whenever you punch someone from this point on, you will automatically use the brass knuckles.