Select the target layer in the Layers Palette and choose' Alt+Ctrl+X' (Mac: Option+Command+X)('Filter/Extract') to open the 'Extract Filter Tool'.
The same applies here as with the 'Default Settings' exercise, except, I used the 'Edge Highlighter Tool (B)' 
and painted along the edge of the building from left to right. When complete, select the 'Fill Tool (G)' 
and click once inside the boundary of the knockout area.
Tips:
1. Shortcut keys work in this panel,
2. So Ctrl+ Plus (Mac: Command + Plus) key to zoom in
3. [ key decreases the size of the brush / ] key increase the size of the brush 4. Use a small brush size
5. If your highlight lines are too thick, hold down the 'Alt' (Mac: Option) key to erase unwanted transition areas.
6. 'Alt+Backspace' (Mac: Option + Delete) to delete the entire edge highlight and start over
7. Switch between the Original and Extracted views before clicking the Ok.
8. The 'Edge Highlighter' works best with sharp edges as you draw your highlight it magnetically clings to that edge.
9. Prevents strays
10. Is not brush size dependent, it will resize it for you based on how far apart your transition is from the interior to the background.
Click Here to view the 'Extract' screen shot.
Here's the final result. (I added a background color layer for contrast)
More Tutorials: 
