Modifying INSTALL.CFG
Open your Install.cfg file with Notepad located in the Thief Gold directory (Right Click on the file and click Open With... Select Notepad and click OK). It should look something like this, Your drive names may be different, replace "C" with the letter of the drive your game is installed on:

cd_path E:\
install_path C:\Games\Thief
language english
resname_base C:\Games\Thief+E:\thief
load_path C:\Games\Thief+E:\thief
script_module_path C:\Games\Thief+E:\thief
movie_path C:\Games\Thief\movies+E:\thief\movies

Erase the+E\...etc on all lines (or whatever letter your cd drive is). Remove the cd path line completely

install_path C:\Games\Thief
language english
resname_base C:\Games\Thief\RES
load_path C:\Games\Thief
script_module_path C:\Games\Thief
movie_path C:\Games\Thief\movies

Be sure to add the RES folder directory to the "resname_base". This important for the next step. Save the file and close it.


CRF Files
In the Thief 1/Gold directory are lots of .CRF files. These contain information that the game uses such as Textures, Music, Sounds, Voices etc.
To make the game more 'Mod Friendly', we shall move these files into their own directory.
Create a new folder in the Thief 1/Gold directory and call it "RES". Find all of the files that have the file extension CRF and move them into that folder.