You seem to be expecting a simple answer, to a question that is extremely broad. The best advice I can give...
---Develop an idea. Something with appeal, preferably mass. Look into many sources for an idea, and then look at possible examples you can base things off of (I.E. say you were going to do a map of China, look at real political and geographical maps of the region).
---After you've found an interesting idea for a map, next comes some of the hard work. Planning out how everything is going to be. How many countries you are going to have, how many continents, if you are going to use impassable borders or split the map into sections, attack routes and borders, bonuses, etc. Pretty much everything that makes a map what it is. Be open to suggestions from people to make your map better and more playable. In this area a lot of uniqueness can be found; try developing different options that stray outside the norm.
---After you've chiseled out the main idea, you need to start to implement it into an actual physical design in some graphics program. Many are out there, quite a few free ones also. A lot of trial and error comes with the graphics stage. Try to get colors with interesting tones and shades. Look into possible continent textures, and the way you want the ocean (or outside border) look. Textures for that also. Experimentation with various fonts, and graphics for bridges or mountains also should be done. And of course along with the actual playing board, a legend needs to be designed to extenuate the work. Every wonderful, beautiful map as an equally beautiful legend. Graphics can be looked into, also sizes and colors of text, etc. Make the legend mesh with the over all map. Again be open to suggestions in this stage to help make the map better.
---Next, once all of the graphics and playability work is done and ready, time to start the XML coding. Many people on the site can help you with it, and there are a few threads and tools that can be used to make the process easier. Make sure to go over the code many times, ensuring no irregularities that could hurt game play (I.E. can't fortify from adjacent countries, or can't attack from adjacent countries, etc).
---Finally after you've done everything you possibly can to make a great and wonderful map for the Conquer Club website, submit it to Lack--hopefully error free.
(Hope I've covered most things!)
So really the process is easy.
Good maps go through many changes in development. Sometimes the longer it takes for a map to be finalized and ready for play, the better. The more polished and shiny it can be the better. The best advice: Don't rush, and be open to another's ideas and suggestions that could make your map better.