I have been doing a mixed media project with my elementary kids and we are having a lot of fun using layers in art so here is a quick tutorial if you want to try it out with us.
First, use crayons and trace or draw shapes in different colors, overlapping them to create new shapes.
Next, add stamp images in different colored inks overlapping and coming off the edge.
Next, color in the shapes with different colored pencils or crayons. Use similar colors, and use the same color in different areas to creat unity.
Add details, words, patterns with another layer with darker markers, overlapping more.
The last step can be collage. Add layers of images over your mixed media background. Have fun make art!