First and foremost, you will need a copy of Skyrim. The mod required does recommend that you have Skyrim Special Edition so if you don't have it click the link down below to purchase it.
https://steamproxy.net/steamstore/app/489830/The_Elder_Scrolls_V_Skyrim_Special_Edition/

For this next part, you will need to download the Skyrim Together mod (and don't worry it's free). Click the link below and you will be taken to the download location.
https://github.com/SkyrimTogether/issues-launcher/releases/tag/v1.0.6

Once you arrive click once on Harbor-1.0.6.exe to begin the download. Once it has completed downloading, run the application so it can finish installing. You may be prompted with a screen that asks if windows has your permission to run the exe, just click yes.
When it finishes installing simply click Finish.

Pretty simple so far right? Well lucky for you it remains simple!
Now what you need to do is run the launcher. Once it's open click INSTALL on the bottom right corner and simply wait for it to install.
If the launcher says "GAME NOT FOUND" then what you need to do is to click on the magnifying glass. It will bring up your file explorer where you must give it a new folder to live in.
Here is the path you need to follow: Program Files (x86)> Steam> steamapps> common> Skyrim Special Edition
Once that is complete click "Select Folder"
If you launch Skyrim through Steam or the mod's launcher and you see this below, that means it worked.

Once you load into the game with a character new or pre-existing just press right control and it will bring up an interface where you can invite your friends to join your server. You can either Join a public server or create a private server for just you and your friends. The easiest way to do this imo is to set up a private network using a vpn. If you don't feel like buying a vpn, then you can get a free one here. https://vpn.net/ (don't worry it is actually pretty good for what it is. A friend of mine and I use it to setup private minecraft servers)

Thank you for choosing my guide and I hope you have fun with your friends in Skyrim! If you have any. I don't :(
P.s this mod is still in development so there are going to be a few bugs.