some basics for all...
Develop an idea/theme , check!
Develop a basic map idea, check!
Decide on how many powers will play and their general starting areas, check!
Unfortunately that's the easy part, but you are on your way.
Now it's time to decide how many starting centers you want/need and their placement may be critical to your theme (for example, if this were WWII you may decide Berlin is a must and you have to place it in a very particular location). This part is also fairly easy
what neutral centers are wanted/needed and where they must be placed is pretty much in this stage of development as well.
But NOW comes the hard part
Time to map things out and determine adjacencies and start developing balance and playability.
What I would normally do is throw in some real basic generic spaces while keeping in mind to maximize conflict and develop as many strategic spaces as possible. Try to max out the number of centers that can be taken in equal moves by multiple powers. Pretty much throw them in and then step back and look at the mess you created. Add a line here, move a line there, erase a line over there, and so on. Step back again and admire your mess and do it all again redrawing the map over and over. When you think it makes some sense, try to put yourself in each players shoes and where you would likely go or never go or possibly go (you get the idea?) try to "play" each power in your mind the first few moves are critical. If your equidistant centers are laid out well, the later stages will usually play along, it's these first turns that are crucial to the rest of the game!
When that's done, run it by someone you trust
But never listen TOO much to others, do take what they say to heart, but you know more and they may not "get it" yes they may and often do point out some things you did not see, but they WILL point to things you know are right, do not attempt to listen to every single thing said, everyone will have a different idea and keeping everyone happy simply will not happen so listen but stick to your guns when you differ.