If I understand you correctly, you want to know how to edit the airport.  Open AFCAD, go to file---> Open Airport, then type the ICAO for your airport.  AFCAD comes with a very good tutorial html file--the readme is one giant tutorial. 
You can run FS and AFCAD at the same time--just minimize one and bring up the other, and the crosshair from FS will show up in your AFCAD.  then  restart the sim and your changes will have taken effect. 

hope this helped?

...meaning create your own, from scratch, fictional airport?  I've never done that, nearly all real-world airports are in the FS in some form, it has 24000 of them.  Shouldn't be too difficult, really.  my advice:  consult the very extensive manual that comes with the program.

As already suggested it'd be a good idea for you to read the very well done instructions that come with AFCAD, but yes, fundamentally you select files/new airport and then insert the coordinates and elevation you want that airport to appear to, then start laying the runway and so on and so forth.

I've done it and it's simple. Only difficulty you might find is to deal with stray autogen. In fact AFCAD allows you to flatten the terrain (both taxiways and apron automatically flatten the terrain they're laid on [I repeat: only taxiways and apron, to flatten the ground under a runway you have to be... creative with the other tools) but does not permit you to generate an exclude area.

To generate an exclude you have to use other tools.

A1 I am making the SimV airport from AFCAD so yes, you can make airports from scratch. Get xfire if you don't already have it and we can work on it together.   www.xfire.com