Download the latest Unity Uploader(https://help.altvr.com/hc/en-us/articles/360015520014-How-do-I-get-started-with-the-World-Building-Toolkit-Unity-Uploader-)
In my case, when I double clicked the file, the pop up to import the package automatically showed up in Unity
AltSpaceVR (Top menu in Unity) > Build Settings
You will see AltspaceVR build Settings pop-up. Log-in with your account
Create a new Template. It will lead you to the website to make a new template.
If you want to see how it turns out as you are building your environment,
1) Click "Load your Templates" Button to choose a template that you want to update. In your case, the new one you've just made.
2) click "Build&Upload" Button. It will update the template with your environment
To use the template to build your world, go to AltVR Website(https://account.altvr.com/worlds)
1) My Worlds (Top menu)
2) Create Universe
3) In the Universe, Create World
4) The template you made will show up here. Select it, create world, then you are good to go.
Enter your world.