Here are some of my favorite pages from my sketchbook last semester!
For this study of the Arctic Hare, I drew the bunnies above on the left side of the spread, then on the right side (below) I cut out bunny shaped holes, spread tracing paper over the holes, and drew their skeletal structures on the tracing paper. When the page is turned over on top of the original bunnies on the blue background, the skeletons become an overlay so I can see where the skeletons fall on the rabbits.
This one was draw on vintage wallpaper.
For these next two, it was hard to show with the scans, but the colored portions of the trees are three-deminsional and pop up off of the page.