We used a recipe I found on Pinterest. The only thing I would say is that it was hard to get the chalk into the Toilet Paper rolls, maybe if it was runnier it would be easier mostly because the Wax paper kept falling in. The plaster of paris firms up fast, really fast. Maybe if it was more runny it would not firm up as fast and be easier for the kids to do. So basically I would add a little more water then what the tub of plaster of paris says. I got mine at Michael's with a 40% off coupon and it was under $5, with plenty to spare.
Here is the site that I got the recipe from please go there!