As I said in the summary this is only compatible with the console verion of this game, the reason for this is because the multiplayer.
How It's Done.
This glich will need an ingame cheat code called Ninja Theme, it gives pedestrians katanas the cheat code is with d-pad XBOX: A, A, Down, Black, White, b, RT, B, X
find a multiplayer symbol on the map somewhere (doesn't really matter which on google their locations) stand over it and press start on the second controller, choose any character for the 2nd player.
Have the 2nd player retreive a Katana from a pedestrian.
The dificult part of this glich is to chop Cj's head of with the second player, you need a clean vertical slice near the head.
Once you do this Cj will be dead. he will respawn near the multiplayer symbol but headless.