I'm not going to say anything fancy. My friend have provided similar instructions on his Facebook months ago; I'm sure that a possibility to create a 1.13 sandbox was mentioned by many other good people many times. But I wanted to put together a very short guide on how to get it done.
Step 1: download the WoW Classic client. The Google Drive is not mine, I just provide the link I found on Ownedcore (thanks starrheld) “as is”. You can most likely skip this step if you have the client from elsewhere.
Step 2: download Arctium sandbox. Then launch it and choose “Classic”.
Step 3: launch the sandbox and the game. You've just downloaded two executable files. Launch Arctium WoW Sandbox.exe. Then copy Arctium WoW Client Launcher.exe to WoW Classic folder you have from Step 1. Try launching Arctium WoW Client Launcher.exe. If it does not work, what helped me is to create a shortcut for Arctium WoW Client Launcher.exe, and in the shortcut's properties, write the path to your client (in my case it's N:GamesWoW Classic BetaWorld of Warcraft_classic_beta_) to the fields “Target” and “Start in”.
Step 4: login and “play”! Email will be [email protected], password will be arctium. Read Arctium's readme for details – you can also teleport, fly and even add NPCs.
I don't think it will help anyone to get the WoW fix, but at least one can set up the addons and or just see Vanilla with some UI/graphical tweaks.