1. General rules for all collaging
Collage Basics
General rules for all collaging

2. Picking Images
All images should be original or so thouroughly manipulated that the beginning picture is not recognized.
Images should combine together into a cohesive whole (unity).

3. Balance and Unity
Overall piece must be well balanced. Pay attention to your values, lines, shapes, and textures.
You do not want one side to seem unusually heavy.
Make sure that all colors, lines, and images flow together in a natural way.

4. Techniques
Layering pictures and papers using modge-podge (or similar sealant).
Paint, charcoal, oil pastel, color pencil, sharpie marker, ink, water color, etc.
Transfers (using tape, or water)
Stamping, spatter, lift off
Layer finished piece with sealant to protect work.
Just experiment!