Step 5: Create a canvas for sketching
Construction of work axes and planes are your best friend for this project. Understanding how the various options can clarify your virtual environment will make your experience less frustrating. I used an offset plane to place my canvas on. For the portrait, I wanted to cast the shadow on my wall, so I needed a plane parallel to the xz plane and 70 inches away. I turned off the visibility of every other component in 123D and then placed my canvas using the .png image created with Picasa.