Prepare your sugar cookie dough according to instructions. (I use Haniela's). You can use a turtle cookie cutter or freehand the shapes. Bake as directed and allow to cool.
Prepare royal icing and tint in desired shades of white, green, blue, or whatever strikes your fancy!
Dilute royal icing with a few dribbles of water at a time until the icing runs off spoon in continuous ribbon (flood consistency.) If you're new to royal icing, you may want to outline your turtle in thicker icing, then fill with the diluted version.
Using a #3 or #4 tip, draw the outline of the turtle and the shell using light green royal icing, then fill so the outside part is filled in and the circle of the shell is empty. Allow to dry at least 6 hours or overnight before continuing.
Wet on wet technique: fill inside of shell with flood consistency icing, then create your patterns while icing is still wet so colors blend together. In the video I used marbling technique, concentric circles, and a scribe tool (though a toothpick works too). It's easy! If you can make circles and lines and you have a toothpick, you can do this.