Start by having the students draw an image on the inside of a styrofoam tray. It's a good idea to have them draw an idea first on paper. Make sure to tell them NOT to push to far into the foam that they poke a hole
Then mix up a large amount of plaster and pour into trays. I used a scoop to pour from a 5 gallon bucket. I did the pouring after school and it took about 35 minutes to do one class. About 5-10 minutes after you pour and the plaster starts to set up, drop an opened paper clip on the back so later the students can hang their artwork. Make sure you put the trays in the "Up" position because you wont' be able to see the artwork to know which way is up.
These are unfinished because we ran out of time during camp, but you can use any type of paint to color. Last fall one of my 3rd grade classes studied Shakespeare so the students drew an image from Romeo and Juliet. Another good prompt is to have them draw their favorite book character. I haven't done this lesson in years, so glad to rediscover it.