Use an APK editor to modify the libcocos2dcpp.so file (or equivalent) to change the server endpoint URLs.
Use a tool like the GDPS Switcher (available via Geode) to easily toggle between the main game and our private server. Geometry Dash 1.1 Private Server
Create a fresh account directly in-game to start saving your progress and uploading levels! Use an APK editor to modify the libcocos2dcpp
Drafting a "deep" post about a Geometry Dash 1.1 Private Server (GDPS) Geometry Dash 1.1 Private Server