Download an illustration of urchins and coral. Take the file to a copy center to have it printed full-size (24 by 36 inches). You can choose a print of your own in a similar size instead.
Using spray adhesive, adhere the print to foam board.
Since you will not need the shoulder blades, unscrew them from the arms.
Determine placement of arm bones: Place them on foam board, so their elbows are at the corners and hands reach along the sides. Anchor one arm at a time to foam board: Using tapestry needle threaded with monofilament, and starting on the back side, sew through foam board, loop monofilament over bone, then push the needle back through. Knot. Repeat a few times in the same spot. Then repeat this stitching in several other spots along the arm.