IIR, there isn't a full fleged map editor yet (though it wouldn't surprise me if a community made one doesn't spring up soon). Right now, if you are playing off the betas, you should be able to create a map by hand editing the map file with a program that can read/write xml files (a common thing).
To start take one of the map files and make a copy of it. You should find the maps in data\custommaps folder.
Now name the new map file to some short descriptive name and open the file with an xml editor. Look around and start tweaking some things to get a feal for how to do it this way.
Or if you aren't keen on editing (and later creating) maps this way wait for a graphical editor.
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