Did you go to the definition files area and look around, flicking through each page and thread? Not sure what restrictions mods put on newbs for some of the things uploaded, maybe you might need to make more posts? Either way there are likely plenty of files in there to make a good def file. If not the same ecu code then you need to look into similar ones which should share all or many of the same maps - there can be variances on age/revisions/parts used/model of vehicle etc but lot's can be the same if you look for models/engines similar to yours in age/spec.
Then you need to read up on the documentation, FS/FR are function sheets/funktiosrahmen that teach you about the ecu set ups and maps.
Then the S4 wiki has it all pretty much laid out for you once you sort a good def file out as well as the tuning threads like you mentioned;
https://s4wiki.com/wiki/Tuning