Select "Game Options" at the main menu, then choose the option to enable the console. Press ~ to display the console window then enter developer 1 as a command to enable cheat mode. The "Load" button will now appear. Click it and select the desired level. After the level loads, press ~ to display the console window again, then enter devmap as a command. Press ~ during game play to display the console window.

Effect - Code

God mode - god
Refill ammunition and grenades - give ammo
All weapons, full ammo, health and armor - give all
No clipping mode - noclip
Ignored by enemies - notarget
Suicide - kill
Spawn indicated item - give [item name]
Flight mode - ufo
Teleport to a node - jumptonode
Level select - map [level name]
Mission set select - /seta [mission set name]