Hi,
As Ozzy said you unzip and follow the instructions. However, I will add an small example.
I download a new plane, say : me109.zip. When you unzip it it will likelly ot create a folder of the kind
me109_g2/ inside it there will be four folders and some other files. The sub-folders will be named model,
panel, sound and texture. Do not delete any file. (with some time and experience there are some readme
files that you will learn you can delete).
Copy the whole me109_g2 to a directory named: /program_files/CFS2/aircraft/ (the páth may be slightly
different for you) and now you can use the aircraft in CFS2
You may also need to download: JoneSoft CFS quick combat editor (search for thsi in google) and CFGedit to customize the the panel of your plane (do not forget to make a back up safe copy) to make things running smarter.
Hope this helped. Same reasoning applies to sceneries, etc... With time and patience you will learn all.
Rgds,
Francisoc