First you need at least a heightmap, a texturemap, and a metalmap (though metalmap is now optional, since Zero-K is using the metal spot gadget). This should contain some basic information
http://springrts.com/wiki/Map_developmentThen you need to compile them.
Here's a handy editor program, though its map compilation feature doesn't work on ATI:
http://springrts.com/wiki/Maps:SpringMapEditMapconv is a map compiler that does work on ATI:
http://www.springfiles.com/show_file.php?id=2585When you have the .smf and .smt files you get by compiling, you still need an .smd file, which is a text file containing info about the map. You can just take one from an existing map and edit it, to get started.
Then put these files (be sure that they have the same name) into a folder called 'maps', and use 7zip to make a .sd7 file of this folder.
Then upload the map to springfiles, and go to a springie host and change to your map. Then say !maplink. Now, if links are found, the map should be added to Zero-K map database. If not, you either need to wait (max half an hour usually) and try again, or there's some problem with your uploading.
That's the most basic way to get a working map. If you've questions about specifics, feel free to ask.