The level editor is the tool that should be used for creating and modifying maps. You need to have your account enabled for map editing, by following the steps in this forum thread. The thread gives a general overview of how to gain access to the map editor and how the editor works. Any information that wasn't discussed on the thread can likely be found through this link.
You can add custom graphics, see how many times it was viewed, manage your map's additional settings, such as PvP, GvG enabled combat, and access code through this link. Note that the editor through here is broken.
This link is what you should go through in order to edit and/or create maps. You will have to log in first.