Please note - you need to be a registered developer to utilise these packs.
First things first - PC & Mac users, you need to install Unity version 2017.2.3 (not 2017.3 or later) - get that from the Unity Download Archive. Linux users, you will want to go to this page and grab 2017.2.1f1 from post #57.
Next, grab the Editor Pack
If you are upgrading from Editor Pack 12 or earlier, you will need to delete your Assets/SpacePack folder before installing Editor Pack 13p17
Install the Editor Pack using the Assets --> Import Package --> Custom Package menu option.
If you are planning on working on clothing & wearables, you will also want to grab this patch
Install the patch after you've installed the Editor Pack using the above process
After the Editor Pack has been installed, you will either need to install the Native Mac Uploader (recommended) or the legacy mono framework version 126.96.36.1992 (click here to download). Installing the Native Mac Uploader is easy, simply click on the SineSpace --> Tools --> Install --> Native Mac Uploader menu option. This process will take a little time, but should be completely automated and you will receive a message once the uploader has been successfully installed.