Photo-sketch : I started Faunagrad with a bunch of cut out photos and flipped and scaled them until I found a pleasing composition. This is also helpful to figure out how many layers of depth I will need.
Detail: This is an isolated detail of the flamingo in the foreground. As you can see, it is constructed out of various objects. I spend hours staring at Google Image Search, looking for anything that can be easily reduced to a silhouette (objects ranging from a mailbox to a chastity belt). This image is at about half the resolution of the original (to give to an idea of the level of detail). Each animal is fully rendered, even if it is in the back and obscured by other animals.
Color: Once I have all the animals constructed, I experiment with color. I usually use a photograph as a starting point. Ones with strong atmospheric perspective work best. I took this particular photo in Brighton, Michigan at dawn.