Paint the styrofoam ball brown and allow it to dry. The thin layer of paint will represent the cell membrane.
Slice the styrofoam ball in half using the knife. The exposed surface of the foam represents the surface of the cytoplasm.
Carve a 2-inch diameter hemisphere out of the center of the exposed styrofoam surface. This is where the nucleus will go.
Make a ball out of black clay with a diameter of about 2 inches; this is the nucleus. Layer purple clay over it to represent the nuclear membrane then cut a quarter out of the sphere to display the inside of the nucleus. Make a 1/2-inch diameter sphere out of dark blue clay and cut a quarter out of it. Insert the blue quarter sphere into the missing quarter of the nucleus to represent the nucleolus. Make many small, evenly-spaced holes in the surface of the nuclear membrane with a toothpick.
Anchor the completed nucleus into the cavity in the styrofoam sphere with one or more toothpicks such that one of the flat sides of the missing quarter is parallel with the surface of the styrofoam.
Mold a long thin "rope" out of green clay and, starting at the nucleus and ending at the cell membrane, fold it back and forth on the surface of the styrofoam so that it resembles endoplasmic reticulum. Make another two or three of these, adding small red balls of clay (ribosomes) to a few of them to represent rough endoplasmic reticulum. Mold some more of the small red balls and place them directly on the styrofoam to represent mobile ribosomes.
Make another "rope," this time out of yellow clay. Mold a flat, red, oval shape about 3/4 inches long and fold the yellow clay back and forth on top of it to represent the densely-folded membrane inside of mitochondria. Make a few more of these and stick them to the surface of the cell.
Lysosomes and vacuoles are small half-spheres of purple and light blue clay respectively that can be attached to the exposed styrofoam with toothpicks.
Make small bunches of segmented orange ropes laid side by side to simulate the appearance of Golgi apparatus.
Finish your cell by fashioning a pair of centrioles out of two short segments of licorice twist. These should lie at right angles to each other near the nucleus and can be attached with toothpicks.