Find an image of Spinosaurus. A good place to look is in magazines and coloring books. Cut the image out and enlarge using a copy machine if desired.
Purchase a rectangular piece of Styrofoam that is large enough to cover the cutout dinosaur. A local art store will carry Styrofoam, usually in the florist section.
Place the cutout dinosaur on top of the Styrofoam and trace with a pencil. Remove the cutout.
Cut around the traced outline using a bread knife and then push a craft stick in the middle of the stomach on the bottom edge.
Add texture by applying a concrete patch mix, found in home and construction stores. Use disposable gloves and a craft stick to spread on a 1/2 inch thick layer. Let it dry.
Paint as desired using a paintbrush or sponge. Let dry.
Protect from the weather by applying a concrete sealer.