My grandson, Zack, has a store bought TMNT costume for Halloween and this is how I made a more realistic shell to take it up a notch.
Here is how I made a Teenage Mutant Ninja Turtle shell
Start by cutting an 18" x 12" oval from cardboard.
Create shell design. First make the large hexagon shape (#3). It's 9" wide and 6" tall and the bottom of it is 3.5" up from the bottom of the oval.
Next, make another hexagon (#2) a little smaller...(9" x 5") Once you establish these two pieces (shown in pink), all you have to do is draw a line out from every corner. (shown in purple)
You will have four sections down the center, and three sectionss on each side. Number them. Use xacto knife to cut along the lines. (Photo A)
Create a second set of shell pieces by tracing all 10 sections on another piece of cardboard. Number them. Cut them out, making them smaller than the first set.
Use hot glue to attach the first set of sections together creating a bit of a mound as you go along.
Hot glue the second set on top of the first set. Because they are smaller, it will create spacing between the sections. (Photo B)
I created a battle scar by cutting into sections 2 and R1. You can personalize your shell with as many scars as you wish.
Cut out random shapes and glue around the outside edge of entire shell. (Photo C)
Final step before painting is covering the shell with paper mache.
Make sure it's completely dry before painting. (I waited several days)
Painting The Shell
Use acrylic paints. Paint entire shell with a basic tan color.
Paint inside all the crevices with black/dark brown.
Use a dry brush technique all over the shell using dark brown.
Dry brush using dark green. Dry brush a small amount of bright yellow for highlights.
Keep adding green, brown, and small touches of yellow, as needed.... blending until you have the desired look.
Finish with a varnish top coat.
I cut up an old backpack and glued the part with the straps to the back of the turtle shell.
I then covered the back with cardboard, after making slits for the straps to come through.