Create a new layer and use a very soft brush to apply the fog where it is needed (more in the background than the foreground). Use the brush at low opacity say 3% to build up slowly. In order to see the fog and place it where needed, it tends to be overdone but this is OK because you can then reduce the opacity of the fog layer. Do not use pure white for the fog. I usually find an off-white colour in the picture which seems to work better. Here is an example:
