Extract the zip file to \PAYDAY3\PAYDAY3\Binaries\Win64\Mods (or \PAYDAY3\PAYDAY3\Binaries\WinGDK\Mods). Make sure that the parent folders aren't repeating, for example: "Mods\Example\Example\(files)" should be "Mods\Example\(files)".
If you aren't using Mercury Mod Manager (or the game crashes):