Type them in whenever you need them:
 
 idbehold- Displays menu (followed by S, V, I, R, A, or L
 for choice)
 S=Strength (Berserk)
 V=Invulnerability
 I=Partial invisibility
 A=Full Automap (computer map)
 R=Anti-radiation suit
 L=Light amplification visors
 
 idchoppers- Gives you the chain saw
 idclev- Warp (followed by episode number and level number)
 iddqd- Degreelessness mode (God mode)
 iddt- Toggles Automap between normal, full, and full with
 objects (enter when in Automap mode)
 idfa- Gives full ammo, 200% armor, and all weapons
 idkfa- Very Happy Ammo (full ammo, 200% armor, all
 weapons & keys)
 idmypos- Displays your bearing and coordinates in hex
 idspispopd- No clipping (you can walk through walls)