After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Move the main folder of the mod to assets/mod_overrides. The assets folder is supposed to be in your game's installation folder (don't know where that is? click here for PAYDAY2 folder). Create the folder if necessary
TO INSTALL: drop the "Secret Tablet English Translation" folder inside your "mod_overrides" folder located at steam/steamapps/common/payday 2/assets/mod_overrides.