The best and simplest example of a working
GuiderInterface is the
ArduinoST4 driver. This class is clean and minimal and adding an explanation here would be redundant.
The only thing implemented in this class not covered by the tutorials so far is the timer functions
IERmTimer, but they should be pretty straight forward to understand. They are just adding/removing delayed callbacks to functions.
If you want to know how to implement a guider driver, look at
arduino_st4.cpp in the main INDI library.