Masking isn't that tricky, but then again been chopping for about 9 years and doing and teaching are different so you're probably right.
Here is what I did, very brief very quick.
1)Duplicate the layer 3 times
2)Set 2 of those layers to colour burn mode on top of your duplicated normal mode layer
3)Merge the three duplicated layers.
4)Make a mask on that newly merged layer
5)Go into the channels option (next to the layers pallet) and select the blue layer hit CTRL+A To select all
6) CTRL+C to COPY IT
7) Go down to the blank alpha channel (from creating your mask) select it, hit CTRL+P to paste and then CTRL+I to invert.
8) All that is left to do is go back to the layers channel and there should be one completely masked out sky around a blackish building.
9)Drag the actual mask onto the original layer you opened with. and delete the duplicated layer
10)you should only be left with one layer the original layer with a perfect mask on it now.
11)Set a new background layer to a light grey to match the foggy sky
12)duplicate the newly masked new layer 3 times
13) set the top to multiply
14) set the middle to Soft light
15) leave the bottom in normal
your haloing should be gone and your foggy sky should be matte.
In theory but who needs theory when I have it here:
[link]1 Minute masking job.
This is what he wanted right?