Currently (in FG 2020.x) the recommended way to download aircraft is to do it via the graphic laucher. This way you ensure that the version of the aircraft is matched to the version of FG you have. I honestly don't remember if that method is present in FG2018.x.
If you want to use the old method for some reason, the aircrafts subdirectories go into 'fgdata/Aircraft' directory. Locate it and just unzip the new aircraft archive there. To tell FG to use that specific aircraft use --aircraft= command line option.
If you download an aircraft from github as a zip file, the directory will contain a word '-master' in its name. Just remove this part and keep just the aircraft model part of the name (e.g. if you got Cessna 182 this way it unzipped to c182s-master. Rename c182s-master to c182s).
Have fun.