Mod idea - Scandinavia TW for RTR platinum engine

I've been playing RTW Vanilla and now RTR platinum 1.9 for a couple years (mostly as barbarians for the challenge) and got the idea to make something entirely new. My idea is a map and campaign dedicated solely to the native Scandinavian, Finnish, Celtic, and German peoples of the old world. As I do not know much about modding (only how to edit text files, files in imperial_campaign, and map_regions), I checked out some of the tutorials. My first step would be to create a functional, geographically accurate map. I first thought about stretching it from the British Isles (West) to Finland and Northern Russia (East), to Northern Germany and France (South) and the tip of Scandinavia (North). Now these are just rough boundaries, I don't know if I will stick to them, perhaps I will do only Scandinavia itself. Anyway, I looked at some tutorials but could only learn how to edit the existing campaign map, cutting sections of the larger map, etc. How does one create an entire full-sized map from scratch? My other concern is that it is hard to find any detailed information on tribes living in Scandinavia during this time period, does anybody know of a good source to learn about early Scandinavian military, settlements, religion, etc.
If someone can help a newbie modder, I would appreciate it.:bandana: