My guess is as good as anybody elses, but here are my assumptions anyway.
Start with a large enough map. The first beta map was 128x128 miles, the current MA is 256x256. The central mediterranean SEA is 512x512 miles. Personally, I'd like to see a tight action packed 128x128 again in the MA. Imho, a 512x512 map might be a bit too big for MA use, but I'm sure there are many who disagree.
For MA use, the terrain should be suitable for the 3-country war. Whether you do it by placing the fields in a certain way or by the terrain (like in the beta map), is up to you I guess. Once you have decided what kind of a terrain you're going to have, print out a sketch of the map and plan how you're going to align everything there.
Fields should be positioned carefully, with the vehicle spawn points thought out. Also, combining the strat objects into the map is important, so that all countries have a level starting point after the reset. So that one country doesn't have all fields elevated while the other countries' fields are on a large plain. But on the other hand, a terrain like that could be interesting.
All countries should have maybe about 10 airfields and then vehicle fields in the mix. And don't forget the ports if you have water in the map. Strat targets include city, HQ, fuel refinery, training complex, ammo factory, radar factory and flak factory. The limit for the fields is 64 for one terrain. So thats a maximum of 21.33 fields per country. The maximum amount of objects is 4000. Including all airfield-, strat- etc objects.
And the most interesting aspect of the terrains is of course the CLOUDS!
Read my instructions how to do them here:
http://bbs.hitechcreations.com/smf/Smileys/default/Forum55/HTML/000090.html The amount of clouds in the HTC terrains has been 256. So that might be a good rule of thumb how many clouds to have in a terrain, visible at one time.
Then there's your favourite subject - textures. They should look good while being good for gameplay. Photorealism isn't something to look for, but a realistic enough look in the game, imho. If it looks too "polished", thats not good.
Once you've finished the map, its ready to be submitted to HTC for last minute checks and evaluation. I guess it is best that you really get the map finished and play tested in a closed circle before submitting it to HTC. When its 99% done, I'm sure they're more eager to help finish it.
Making a map is quite a big project. But after you're finished the terrain work itself and decided where to put the fields, it will be quite straightforward just to slab the objects there. Especially if you're using the default MA airfields and strat object that come with the editor.
Good luck!
Camo