First, my husband wore a long sleeve green shirt and green pants for this turtle costume. He painted his face green. To make the shell we used a thin wire (like for electric fences) and formed a frame for the shell.
After the basic frame was made he taped small sections of cardboard in each of the squares we made with the wire. Finally, we spent several days using paper mache to cover the shell. Then, we painted the shell.
For the front piece we cut out a large piece of cardboard and painted it. To keep both pieces on we used elastic to make straps. It took a few weeks for everything (we did a little each day) and about $15 because we already had most of the supplies lying around.