Here are the instructions from their web page
Installation instructions
The overwhelming number of potential problems reported are related to installations. The German Beta has some flaws we missed (you might want to wait for the translators to catch up, some things can cause crashes). The following are stalker proof installation procedures:
- Uninstall any mods from the past
- Uninstall the game
- Delete the entire folder your old COP installation was in
- Reinstall the game (S.T.A.L.K.E.R. CoP)
- Start the game – configure best for your system – save once (make sure it is running smoothly)
- Unpack the mod, Please use 7zip to avoid any chance of file corruption during unpacking.
- Right-click on the MISERY setup.exe and run as administrator
- Choose your options
- Pay attention to the note at the end
- Start a new game
You DO NOT need to edit any files to enable the mod. We want you to be miserable playing, but not with unplanned installation complications. Skipping any step (verifying your game cache in Steam is NOT the same as reinstalling – it will leave dregs of former installations which can cause problems) will dramatically ruin your experience. Note: If you use steam, permissions on the .exe GSC made in July will make removing the old .exe impossible until your close steam and reboot your computer) then you can remove it so the installation goes smoothly.
If you have further questions let me know. If you have any questions about the install options that come up please let me know that as well.