Basic editing is easy. Simply click the map tiles and place them. Add a spawn and click Setup Default Players. To make the green fields look more random click Fix Open Areas. Click Properties and add a small description. Be sure to set [x] Show in map chooser. Save the map (no spaces if you want your map shipped in the official distribution). When resizing the map use the up and down arrows to be sure to only use allowed values (dividable by 8).
OpenRA comes with an easy to understand map editor. It has support for importing legacy maps. You can also create new maps from scratch.
Posted by Milanium on - Basic Mapping/Technical