How do I install a newly downloaded plane?

From FlightSim

In the main FS2K Aircraft folder set up your xxx plane folder with four sub-folders: Model, Panel, Sound and Texture. Put the xxx.air file and the aircraft.cfg file under the main folder and the others under the appropriate sub-folders. Using MS Explorer, look at other planes for similar construction. If the zipped files have their folder names included, Winzip can place them automatically. "Flightzip" and "JAB2000" are two utilities available here that help place the files. Any FS98 guages called out in the Panel.cfg file must be copied to the main FS2K Gauge folder. If the plane was converted for FS98, it should not be converted again. Some of the addon planes are simply repaints of the default planes. That is, only new Texture files are included. Save backups of the default Texture files in case you don't like the new livery, then install the new Texture files and try them out.

Also, check the FS2K Forum using the keywords "Download", "Addon" or "Installing new aircraft". Then, read Q&As 3, 6 and 32 under Aircraft in the FS98 FAQ, which has about the same "Aircraft" file layout as FS2K. These give many details.