For the first step you'll need to know how to open TF2's console and create binds. Skip this if you already know how to do this and have more than 5 hours in the game.

• Go to options under the main menu. Under the keyboard tab there is a button that says "Advanced..." Click the button.
  
• In the new window check the box that says "Enable developer console"
  
• Close the options window and press the tild key ( ~ ).
  
• Congrats, you opened the console. Hopefully.

The format of a bind is simple.

A large number of community servers have Source Mod installed, allowing them to easily add in mods and various scripts to their servers. Source Mod comes with options for Friendly Fire by default. Admins can easily change this, but for simple users a message stating whether or not friendly fire is enabled will appear. The thing is, is that on most servers that message will be printed in chat server-wide. This is where we start messing with people.

Open the console and bind a key to the command "ff".

From there you can just spam the key and watch as chat floods with "[SM] Friendly Fire is disabled." More imaginative people can get creative with scripts and junk, but this is enough for me. If you're using the command through console, lower level mods shouldn't be able to tell what's going on. Admins or people with access to the server console will, so watch out.

Have fun and if anything is incorrect in this guide, let me know.