I think that a good point to start is to make a list of all known/heard of places.
I am pretty sure that I don't know all the places, so if you folks wanna join me with this I would be very grateful!^^
Here is an example of how it could look:
(Divine Divinity map based)
Aleroth:Since it is the very first location you get to know in Divine Divinity, I would make this the centre of the map
Flussheim (is it the same in english?:Capitalcity of the duchy Ferol, located south-east to the Aleroth surroundings
Teneb Tiriel:Forested region in the south of Ferol
Yuthol Gor:Wasteland region. Lays east to Ferol and Teneb Tiriel
Tanoroth:Mountain range to the west of Aleroth and Ferol
Verdistis:Big city in the north of Ferol
etc....
you know what I mean^^
EDIT:
I took a few minutes to sketch out a very rough image of some stuff that I know. Have a look here:
https://www.dropbox.com/s/18vjmhruwrhv0qn/Sketch.png?dl=0
