Get the "DemiLib_Complete.zip" package to get all the DemiLib stuff, or download a specific configuration with only the files you need.
To download, just right-click on a package and select "Save link as..."

• Unpack the content of the ZIP as it is in your Unity project, so that it ends inside a Demigiant directory. You can move it around, but always keep Demigiant and DemiLib stuff inside the same directory, so you won't end up having multiple copies of the same packages lying around.
• If later you want to add more DemiLib stuff, just overwrite the existing DemiLib directory in your project with the one you'll find inside the ZIP.
• If you have DOTween Pro in your project, and you're importing a configuration that requires DemiLib core, first delete (not simply overwrite) the DemiLib folder that DOTween Pro already put in your project, since it's an old version with a different directory layout.