I draw it a bunch of times on paper. Then I draw it in paint. Then I draw it in the terrain editor. Then I'll mess around with the jpeg. Then I'll make it again in the terrain editor.
it helps me alot of have a picture of the map that is the same dimensions as the jpeg of the map, so i can be more meticulous with the drawing of the map, and then when I make it I can have a better reference and it makes it easier for me to do the terrain because i suck with making terrain.