To install the Render Boss Extension manually, just download the following zip file:
Unzip its contents to their respective subfolders inside the Scripts directory in the After Effects user folder. This folder is usually located at:
Windows: C:\Users\{USER}\AppData\Roaming\Adobe\After Effects\{VERSION}\Scripts\
MacOS: /Users/{USER}/Library/Preferences/Adobe/After Effects/{VERSION}/Scripts/
Restart After Effects, and the panel should be available inside the ‘Window’ menu.