Instructions
Enter a game that is using the No Quarters game mod. A multiplayer server using this mod will have it listed in the server description, so you will know before you join.
Open the console by pressing the "~" key, right next to the number one on your keyboard. This will open a chat-like box on your screen, showing you other client-to-server commands that have been enabled.
In the console command box, type "/bind [key] playdead" replacing [key] with whichever key you want the ability bound to. This key should be in a convenient location to use during gameplay; but not already used for another action, such as reloading or switching weapons.
Press "Enter" to accept the bind command. Press the key during game play to use the Play Dead ability.
Repeat steps 2 through 4 to change the key bind again, if you find you do not like the previous setting.