Nemo and Squirt are made from piped cookie dough and crushed Life Savers (Jolly Ranchers or other translucent, hard candy of that sort would work just as well but if I remember correctly, I chose the Life Savers because they were easier for me to crush). The sea floor was made from melted chocolate and the ocean water was created with melted marshmallows (dyed blue with food coloring).
Here are pictures of Nemo and Squirt before they were baked:
You can see that I laid them out on aluminum foil from the get-go (remember the nonstick spray beforehand) and baked them that way. After they were baked, I carefully peeled them off the foil and placed them on the backdrop before the marshmallow had fully hardened so that they were anchored.
Of course, along with the cookies for show, I also baked her some proper stained glass cookies for eating!
I remember the first time I was introduced to the idea of making "stained glass" in the oven. I was in third grade and my teacher brought an amazing-looking gingerbread house, complete with stained glass windows, to school. I was definitely inspired!