Now this is a really fun exercise!

 

What I did was take the photo and flip it upside down.  Then I squinted my eyes and just looked at the different shapes, values (light and dark areas) and angles and started to fill in what I saw.  It took be about 7 minutes all together.  I love doing this with landscapes.  It helps me loosen up and not focus too much on detail. 
I'll post the photo with the picture (upside down, of course!) next to it so you can compare.