If you're looking to help out contributing and adding your feature on it, fork first the repo. Then you will have to clone, first make sure that Openplanet is installed on your game (Trackmania or ManiaPlanet), then you will have to clone the repo to:
C:\Users\%username%\OpenplanetNext\Plugins\for Trackmania Next (2020)C:\Users\%username%\Openplanet4\Plugins\for ManiaPlanet (Trackmania²)
⚠ If you're cloning on Trackmania Next, make sure that you're running the Club edition of the game. Read more here