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)