You must first finish all mission objectives in the single player game before the cheats become available. Then, during gameplay, press "back". After that, ress the following buttons to activate the cheat:

High Pitched Voices:X,A,Y,B,X
Big Heads: A,X,B,Y,A
Chicken Explosives: X,X,Y,A,B
Big Bodies: B,B,Y,X,X,A
Slow Mo: Y,Y,B,X,A
Paper Mode: B,A,X,Y,A