Make sure you can enter the console, you can do this by adding +seta com_allowConsole "1" to the target in the game's shortcut. Then hitting the tilda (~). Or by running the game normally and holding ctrl-alt-tilda(~).

1. Run the game

2. Enter the console

3. Type in "idclip"

4. And there you have it!

Although it says "unknown command" above it, it will say "your memory serves you well!" Idclip was the command for noclip in DOOM II. Idclip is not the command for noclip in DOOM 3, it is just plain old "noclip".