Sketch your design on paper. Your casserole dish may be round, oval or rectangular, with or without handles. Choose a color scheme for your glazes. Mix up a bag of powdered clay, using a drywall mixer. Follow the manufacturer's instructions to get the proper consistency.
Wedge the clay repeatedly to remove air bubbles and prevent an explosion in the kiln. Knead the clay, then roll it into a ball. Cut the ball in half with a knife. Slam the two pieces together. Do this until all the air is squeezed out and the clay has a consistent texture. Keep the clay wet by sealing it in a plastic bag until you're ready to model it free hand or throw it on a potter's wheel.
Roll up a large ball of clay for a casserole dish 8 to 10 inches across. Throw the clay down hard at the center of the potter's wheel so it sticks. Wet your hands and center it. Lean forward and use your body weight to control the lump of clay. Make it spin perfectly, without a hint of a wobble, or your dish will come out crooked in the end.
Keep your hands wet throughout the process. Press down on the clay until you have the approximate shape of your dish. Jam your thumbs into the center of the mound to make an opening. Pull the clay outwards with your fingers inside the dish wall and your thumb on the outside. Make the walls as thin and even as you can. Cut the dish off the wheel with wire and dry it for firing.
Build a casserole dish with the slab method if you don't want to use a potter's wheel: roll out and cut slabs of clay of even thickness and form them into any shape dish you like. Weld the pieces together using water and your fingers. Attach the pieces to a bottom slab. Make handles by using lots of water to get the clay malleable.
Decorate your dish by incising or stamping designs into the clay before it dries. Bisque fire it, then glaze it and fire it again in a kiln capable of 2,200 degrees Fahrenheit.