So… how long is your piece of string?
You’re probably best served getting a grounding in Arduino basics. To be honest, that’s easiest by buying a starter kit, which includes an Arduino board and a collection of switches and LEDs and other components to experiment with. But failing that, you’ll at least need some lights and some switches or buttons or similar.
Go to https://www.arduino.cc/en/Guide/HomePage to get started. I exclusively use a workflow based around installing the Arduino IDE locally, but you may prefer to use the online editor to work on code. Once you have an environment set up to write and upload code, check out https://www.arduino.cc/en/Tutorial/HomePage and work through some tutorials to get a feel for writing code in general, and using different hardware devices with your board. At an absolute bare minimum, understanding the BlinkWithoutDelay and Debounce sketches and being able to write and expand new sketches based on the concepts in them are vital for reading button input and driving LED outputs.
There is no one-size-fits-all Arduino sketch for controllers because there is no one-size-fits-all controller. But there are a couple of libraries in development. I’ll shamelessly plug my own library here: Arduinos In Space: Arduino library for OiS controllers. It has a few examples for trivial setups, and is hopefully fairly easy to expand for larger controllers. Segwegler has also written a library that may be useful, check out https://github.com/Segwegler/OIS_Library
Either of those options will also need a little bit of an understanding of the serial protocol, as well as the commands and data requests that are possible. I’d highly recommend reading the documentation linked from the wiki, under https://oiswiki.sysadninjas.net/wiki/Category:HardwareInterfacing , to get a feel for what’s possible. That will hopefully give you some ideas for how to expand your controller once you’ve got the basics running.