Get a bag of clay to use for your sculpture.

With your clay, get a small bowl and put some clay pieces and water in it. In a couple of days it could look like figure 1.

Figure 1: Slip

Now, take a slab of clay and throw it down on a table. Don't throw it too hard. This method makes the air bubbles come out of the clay so it doesn't explode when you're firing it. Throw it on the table until it is in a cube shape.

Now, take a rolling pin a flatten out the slab of clay. Don't make it too thin or too thick. It should be about 1/2 of an inch tall.