Put on a mask and gloves to protect yourself from the cement.
Pour equal parts of cement, perlite and chopped hay into a large trough. The amount of each will depend on how many flower pots you plan to make and how big they will be.
Pour water into the trough and mix with a shovel. Add water until the mixture is approximately as thick as cottage cheese.
Arrange one or more drainage plugs in the bottom of the cardboard box. You will remove these after the cement has hardened.
Dump enough of the hay and cement mixture in a cardboard box to make a layer 2 inches thick on the bottom of the box, or to the same height as the drainage plugs. Smooth the top with the trowel to even it out.
Place a second smaller box inside the first box. The second box should be sized so there are about 2 to 3 inches between the boxes on all sides except the top. Fill the second box with sand or another heavy material to help it hold its shape.
Fill in the space between the two boxes with more of your cement mixture. Push the cement down lightly to get rid of air holes. Smooth the top edge with your trowel, and round it off if you would like.
Cover the pot with a sheet of plastic and let it sit for at least 24 hours, or until the cement has set enough to hold its shape.
Empty the sand from the inside box and carefully peel off the entire smaller cardboard box and the sides of the larger box. Carefully pull out the drainage plugs in the bottom of the pot. Texture the outside of the pot by scraping it with a wire brush or similar implement.
Cover the flower pot with plastic again and let it set for about a week. Sprinkle with water every few days to assist in the process.