Create a New Profile with the name: reb00.
After creating the Profile, select any mission you like and then when the game/mission has loaded, press the ESC key to pause the game. Now You can see that there is an option 'Cheats' along with other options like Save, Load, Quit Mission etc.
Click on the 'Cheats' option and there you will find cheats like GOD Mode, Infinite Ammo, Silent Mode, Invisible Mode, Infinite Projectiles. These can be Toggled ON or OFF.
Activate the options which you want and click on the Accept Button and start playing the current mission with these cheats activated.
(supplied by: KingDaniel)