After creating the design in Photoshop, I often build a quick prototype of the structures out of paper to help me visualize how things will fit together in 3D. I have a house model that is 1 inch : 1 foot and create miniature paper scenery pieces to fit the space.
One of the fun challenges for this year is the forced-perspective New York City street which will have the Stay Puft Marshmallow Man towering over it. To figure out the design, I began to build the forced perspective structures using paper and wooden skewers.
Then, I took a picture of the structure at the equivalent height of the eyes of a visitor to the house and brought that picture into Photoshop, where I interpolated the horizon and perspective origin point and extrapolated the angles of the tops and bottoms of the buildings.
From there, I cut the sides of the building in paper and I re-tested the forced perspective to make sure it would create the desired effect.