It can be done quite easily. The software used depends on what FS version you are using.
For FS2004: (the same as suggested by Hager)
Afcad 2.21-Lee SwordyFor FSX: both of these require that you have the SDK, included with FSX-Deluxe, installed.
Airport Design Editor-Scruffyduck SoftwareFSX Planner-zBlueSoftwareAll three programs are freeware. Each provides a 2D cad style window to create or modify the visible (runway, taxiways, aprons, parking, surface lighting, etc.) and invisible (comm and nav aids, etc.) aspects of an airport. The FSX programs can place some additional items specific to FSX airports.
If your using FSX-Standard (no SDK) I only know of one airport editor the does not require the SDK:
AFX-Flight1 Software-
payware, demo available....for FS9 and FSX.
Dave
At that time [1909] the chief engineer was almost always the chief test pilot as well. That had the fortunate result of eliminating poor engineering early in aviation.