Look at a number of images showing starfish and select one that you like most. Place the picture near you so that you can look at it frequently while you work. Knead the clay a few times to give it a good consistency. Roll the clay into a ball with your hands. Try to make it perfectly round.
Place the ball on the wood board and flatten it a little to secure it in place. Use the roller to roll the clay from the center out toward the edge. Keep the center a little thicker that the edge. Keep rolling until you have the right thickness for the starfish. Mark the tips of the five arms and where they are located around the center using the needle tool. Outline the shape of each arm.
Cut out the sections in between the arms using a knife. Refer to the images to get the correct length and width of the arms. Continue until all sections are cut away from the arms.
Soften the edges along the cuts with a damp sponge. Rub the sponge along the length of each arm toward the tip. Do this on both top sides of each arm until it looks like the real thing. Add curves to the arms to make it look alive. Use the knife to make a slight undercut under the arms to add realism.
Add some texture and contours to the starfish to complete it. Let it dry for a day until hard.
Paint the starfish in layers. Paint a base color, such as dark red, and add more colors as you see them in the images. Paint the speckled color patterns using a small stencil brush. Keep detailing until you are convinced it will make people wonder if it is real when they see it for the first time.