first, you take a screenshot of the map.
I'd suggest doing it in the map editing window, as you can zoom out to see the whole map, and not have to do puzzle editing to get the big fog together in your photo program (I use gimp)
Next, you create a gradient in a new layer over the map. It has to be a circular gradient, from black to Alpha- 0% black, to create the proper fade.
then, delete the lower layer (the map) and save your completed fog
I'd suggest naming it the same as the map, and putting in the map number, just so you remember.
in a parallel process on the map in game, put a change map event, and make the fog activate
if you were at 1/1 in the map editor when you screenie'd the map - fog is at 100%
if you were at 2/1 - fog is at 200%
if you were at 4/1 - fog is at 400%
(this is the fog's magnification, btw)
coordinates: top left orientation - 0,0
it should end up looking something like this (only with your map, not mine xD)
