During the game bring up the console with the ~ key then type in LOGIN developer. Now put in a code below. Put the code and then 1 or 0 like NOCLIP 0

NOCLIP 0 or 1 - Turn clipping on/off
GOD 0 or 1 - Invincibility
FLY 0 or 1
UAMMO 0 or 1
ENABLE COUNTER - Show Framerate
GIVE_ALL - Get all weapons & Ammo
ENABLE CROSSHAIR - Show crosshair
ROTATE CROSSHAIR - Get next crosshair
LOADSCENARIO("UMC_SPSCENARIO1","Level") - Load "level". Substitute "Level" with a level below

bootcamp
cube
vertigo
brooklynsp
illegal
station
warehousesp
tucson
border
mansion
house
haunted
temple
gothic
space
plant
deadend

NOTE 1: This must be done in the following way or the level will not be playable:

1) Start a new single-player game.
2) After the Bootcamp level loads, do not click the "Next" button.
3) Call up the console and enter the above LOGIN and LOADSCENARIO cheats.
4) When the loading is finished, the console will disappear.
5) Click the "next" button and the level will start.

NOTE 2: Another way to start at any level is to edit the file UMCGAME.LGS in the GORERULES folder under the game install folder. Be sure to backup this file first. Simply delete all of the mission desciptions before the one at which you wish to start and save the file. Each description will look similar to this:

mission1:

spawnCode("UMC_Mission0")
displayPage("umc_m1")
loadScenario("UMC_SPScenario1","bootcamp")
displayPage("next")
pause()
selectPlayer("player")
playScenario()
displayPage("mission_complete")
hidePage("mission_bootcamp01")
pause(4)
debugPause()

There is no need to renumber the remaining mission descriptions. Now run the game and start a new single-player game. The game will load the first mission description it finds in the edited file