Download the plugin
Visit umbra.io/downloads and download the Revit Add-in
Run the installer
After running the Installer, Windows warns you of the dangers ahead! You need to click More info to be able to run the installer.
Now run through the installer.
N.B. If you change the default install location, Revit might not detect the Add-In when you run it. You can always re-run the installer and re-install in the default location.
As you run Revit, you get another warning. Simply choose "Always Load"