They start appearing further away from map center like the guy above said. They generate just like all the previous biomes—patches here and there. Some bigger, some smaller.

I'd only go east/west though and not north/south. Don't reveal Ashlands by going too far south or you'll have a harder time when that biome gets updated.

No. Just blindly ramming your face into fog of war. Theyre generally located in the outer 50% of the map circle. You can kind of see mistlands from a distance from a wall of purplish-gray cloud.