UselessFodder (love his handle, btw :D ) has put together a basic tutorial for using the Arma Reforger editor.
He also includes links to a whole bunch of resources (https://hq.uselessfodder.com/p/how-to-make-missions-in-arma-reforger.html) that he either dug up or created on his own as he was researching the topic. Well worth a look.