Open the Steam Client->Open the Launcher->Choose your favourite Settings->Close the Launcher (Optional)->Load the game via the official game shortcut (not the PLAY button of the Launcher)
-The mod manager appears
-Tick only the DarthMod: Shogun II mod pack you wish to use, one of the below:
darthmod_shogun_vanilla_hand_garr.pack (Vanilla Garrison & Vanilla heavy AI cheats)
darthmod_shogun_vanilla_handicaps.pack (Vanilla heavy AI cheats)
*Note* The launcher saves your settings. So, unless you want to change the settings, you may never use the launcher again and play the mod using the official game shortcut only.
*You have a CTD?* Try this FIRST OPEN THE STEAM CLIENT