As you know, unless you live under a caterpillar-free rock, Eric Carle's books have gorgeous illustrations. Carle uses a multi-step collage process. This process is kid-friendly, and kids as young as kindergarten can produce some very sophisticated results using his method.
Here's some written instructions by Carle on making collages:
Go drag some tissue paper out of the gift-wrap box and get to it!
If you want more of Eric Carle, here's a link to other videos, including Carle making a mural: http://www.eric-carle.com/photogallery.html