Editing thecampaignmap is a complex task, that is hard to explain in general. The map is made up by ceveral "colorcode" maps, that indicate stuff like land/sea, altitude, type of terrain, aso.

Your best bet wold be reading Myrddraal's guide, that takes you through step by step: https://forums.totalwar.org/vb/showthread.php?t=39456

After reading it, i was able to create my map, so i guess you can too.
Have fun

-Skel-