There are two methods you can use to create airfields. The first is by making a new custom airfield plate (somewhat complicated), and the second is by making an airfield onto a texture (less complicated in the end, but it uses up one or more texture slots for your map). This whole topic deserves it's own thread, which I can make in due time, but here are the few tutorials which I did not make:
The texture approach (can also be done to span multiple textures like in my Wake Island map)
http://dl.dropbox.com/u/4435031/Airfield%20Textures.pdfThe plate approach (essentially doing what the tutorials say, but on a grand scale)
http://dl.dropbox.com/u/4435031/Airfield%20Plate%20Tutorial.rarThe most important thing is to make sure your airfield looks good in GIMP or Photoshop before you start trying to bring it into the game as a texture or plate. If your airfield looks crappy, then there is no point in making it, right? Use lots of layers. Also, try to get a reasonably accurate idea of how big the airfield/runways are in real life before you start. It's been a while since I've made any airfields, so I'll try to put together a detailed method of how I do it when I get some free time.
***Make sure all runways have their main axis running on a 5 degree interval on the compass (i.e. a 05-185 runway or a 90-270 runway). You can check this in GIMP by using the compass tool down the centerline of the runway, or by making a rectangle selection and rotating it to the desired direction.