Its a huge learning curve, lots of You Tube videos about it.
However what I discovered on my own s there is some sort of "cheat sheet" foe the variable definitions to lookup for each aircraft. Also if you have the FULL version of SPAD - you can detect what variables the cockpit buttons and levers are actually using - so you get a leg up on what you are looking for.
When you actually assign a button on your setup to the cockpit button, you either send an event usually using SIMCONNECT, or change the data value. Those are the two events that are used mainly in the actual assignment.
I am currently trying to assign new buttons for A320 V2 and having 0 luck.