The easiest way to start the editor is by typing editor
in the console. If you open the editor a lot, you'll probably want to create a direct short cut to it.
1: Right-Click the Doom 3 short cut and select "Copy" then "Paste" it in the same place (so you get "Copy of Shortcut to Doom" or something similar).
2: Rename this shortcut to "DOOMEdit"
3: Right-click the new shortcut and select "Properties". Specify the following for "Target":
"C:\Doom3\Doom3.exe" +set r_fullscreen 0 +editor +set com_allowConsole 1
(This of course assumes you have Doom 3 installed in C:\Doom3\)
The editor does not work with antialiasing turned on, so you may need to add:
+set r_multiSamples 0
The first time you open DOOMEdit, you will want to click File->Project Settings and change all the paths to point to wherever you have Doom 3 installed (by default they are set to where the level designers here at id have Doom 3 installed). There is a small bug in that dialog box, after you click OK, you will need to copy the "doom.qe4" into the "base" folder.