Using a knife, carefully cut the styrofoam ball in half. If necessary, use the knife to make the flat surface of one of the halves an even plane.
Color the outside of the ball with a red marker to indicate the cell membrane. Alternately, you may choose to glue a piece of ribbon around the outer edge of the ball to represent the cell membrane.
Color the flat surface of the styrofoam ball with a pink marker. This indicates the cytoplasm.
Create the cell's nucleus out of modeling clay. Glue it to the center of the flat portion of the ball.
Select other craft supplies to represent other of the cell's organelles. You can use beads for ribosomes and bent pipe cleaners for endoplasmic reticulum and mitochondria. Glue these pieces to the flat surface of the ball. Alternately, you can fashion the organelles out of various colors of modeling clay and glue them to the ball.
Create a key or labels to point out each organelle to make your model easy to interpret.