Add the '-console' command line parameter to the end of the 'Target' field in the 'Shortcut' properties that are used to launch the game. Then during gameplay press the '~' (tilde) key to display the console window and type one of the following case sensitive codes (without the quotes) and press 'Enter' to enable the corresponding effect.
God Mode: Type 'setv cl_superman 1'
Disable God Mode: Type 'setv cl_superman 0'
Infinite Ammo: Type 'setv c_noammo 1'
Disable Unlimited Ammo: Type 'setv c_noammo 1'
Appear Disguised to Enemies: Type 'camouflage'
Ignored by Enemies: Type 'disable AI'
Invisibility: Type 'invisible'
Silent Movement: Type 'silence'
Spawn Indicated Item: Type 'L_CreateItem (item name)'