First, make sure you own game and OST. Don't forget to add free OST DLC to your account!!!

1) Open steam console - use url in your browser steam://open/console and confirm action (it appears in steam client as another tab)
2) Type in command line: download_depot 582620 595980 4797447177801861008
3) Wait until it finishes downloading
4) Check folder C:\Program Files (x86)\Steam\steamapps\content\app_582620\depot_595980
5) Enjoy ;-)

Download proces logs something like:
Downloading depot 595980 (278 MB) ...
...
Depot download complete : "C:\Program Files (x86)\Steam\steamapps\content\app_582620\depot_595980" (48 files, manifest 4797447177801861008)

Just FYI - content is NOT downloaded to Copoka's game directory. You have to check folder in your Steam's installation directory. If you have Steam on other drive or folder then default, you have to check it on other place.