Download and install Domination for PC/Mac from http://domination.sf.net, Start the SwingGUI, and go to the Map Editor Tab.
To start you will need an image that the player will see.
You can open some other maps to get a idea of what things will look like when you're finished.
Here is a step by step guide:
When you have finished making your map and you are happy with it:
submit your map to the MapStore by clicking the "Publish" button at the top of the Map Editor.
Fill in your information, select which categories you want the map to appear in (at least one) and then click "OK".
If you have any problems or need advice email: yura@yura.net