Step 1:
Open ArtTool 2000. From the Edit menu, choose Expand Menu Art File. Select GP2000v1.bin, (located in the GP3 Season 2000\Bitmaps directory) and click Open. In the dialogue box that opens, you must select at least the Expand Team Arts option and Make All Helmets 84x80, then click on Expand Selected Images. Close ArtTool.
Step 2:
In your GP3 Season 2000\Bitmaps directory, rename the original GP2000v1.bin file to something else, (GP2000v1.old, for example). Rename the file new_GP2000v1.bin file to be GP2000v1.bin.
Step 3:
Unzip the 11 bitmaps that make up this Team Art set and the accompanying .tas file into a directory of your choice - I put mine in a folder in the GP3 Season 2000\Bitmaps directory.
Step 4:
Start ArtTool 2000. Choose Open from the file menu and load GP2000v1.bin. Then select Install Team Art Set File from the File menu and select the .tas file in the directory you used in step 3. Click on Open. ArtTool 2000 will import all 11 bitmaps into the game - this can take a moment - wait for the message that says it's finished.
Step 5:
Exit ArtTool 2000 and you're done. I recommend restarting your computer before you start GP3-2000, as mine hangs on me when I try to load a track unless I do this.