@Windrunner21 well do you have triplea downloaded so you can check out all the maps ? If not, you can do that here:
Then search through the maps and pick one you like. Also dl "Pact of Steel2" as that tells you how the game works. Once downloaded go to your triplea folder, downloaded maps. Open up POS2 and keep opening until you see "games".
Open that and then open the POS2 xml with whatever editor you like. Most people use notepad++ if on windows. I don't know much about the graphics part.
Here it is for easy access. It will probably take several readings to get the general idea. Once you pick a map to mod, start small with changing a unit cost or a game note and just kinda go from there.
There's a lot more to it, but if you enjoy learning and are patient, you'll figure it out. Most likely a lengthy process unless you end up as some sort of triplea savant.