This is incredibly broad. I'm going to ignore the FL aspect because it's useless information. Unless you are talking how to physically layer sounds in FL(again this is way too broad). So here is a way too broad of answer
Octaves & filters. Not every instrument needs to be heard to add to the track. Don't look to layer tracks, look to add fullness to your track where it needs it. Don't logically think what it needs, listen to what it needs.
Damn I wasn't broad enough