There are many recipes online and all of them give ratios. I used:
one part Portland Cement
one part Peate Moss
one part sand
one part Vermiculite (not really, it was more like 1/4 or 1/2 of the other stuff)
So when I ordered the ingredients I got the same number of pounds, which was a huge mistake because 100 pounds of the cement was not nearly as many cups or (bowls or whatever you are using to scoop to be considered one part) of Peate Moss, so I ended up with tons and tons of that left over. In fact, for 30 campers, I used about 97 pounds of cement and about the same of sand, but only maybe 1/3 of a 50 pound bag of Peate Moss. Students scooped and counted while wearing their latex gloves and dust masks. You don't want to get the powdered cement in your lungs because the moister is what turns it to mortar and that's not good to have in your body. We used a wheel barrow and worked in groups of 9 students at a time, pouring water and mixing with their hands. You want it to be able to hold together, but not be runny.
Two days later we popped the hardened containers out of the plastic containers and let them finish drying for another day. I think ideally, a week or two would be needed so that it can cure, but a day or two is perfect if you want to carve a design or sand the outside to smooth it out.
Be sure to find a plant that works with your new pot. We poked a small hole in ours, but I'm not sure that's necessary since the Peate Moss should help absorb some extra moisture if the container is large enough. It's a learning process, but so far, so fun!
photo credit for plant pics to Keith Blackwell