Easy, but time consuming... I have done it with 3rd and 4th graders.
Take a circle about the size of a coke can maybe a little bigger. I used my circle cutter shapes from my scrapbooking stuff. I cut the circles out of cardstock.
Cut an inside circle for the pic to show through.
Take green yarn (you will need more than you think per ornament), tie it off, and then start wrapping it over under over under until the circle is completely green. Back it with a full circle and a piece of yarn for the hook. Hole punch red paper for the "berries" and glue a few on. OBTW, don't forget to tape the pic of the kid on the "full circle.
Hope this makes sense. I don't really have a pattern or pic to give you, but pm if you are interested.
The kids have a good time and they look like a bunch of little old ladies knitting at the Senior Center!