First of all, you will need to save the game at the point you wish to start cheating, if you want to start a new game then you will need to sit through the intro and briefing then save the game when you start out in the castle dungeon.
Once your game is saved, quit Wolfenstein and edit the properties of the shortcut you use to start the game. You need to add the text "+set sv_cheats 1" to the end of the "target" line like so:
Once you have done that click OK to save the shortcut and double click to run the game. Once the game has loaded you will need to load the savegame you made, do not start a new game because doing so will disable all cheats.
Notes: When you complete a mission all the cheats you had enabled will dissapear and you will need to activate them again, for example if you were playing with GOD MODE enabled you will need to re-activate that cheat once the new mission starts.
Note: To be able to use these codes you will need to follow the instructions in the "Activating Single Player Cheats" code found on the first page.
To warp to any level in the game, bring down the console by pressing the key (under escape) and entering map followed by a level name from the list below. For example, for the second boss - type spdevmap boss2