Form clay eyeballs. Use white Ultralight Sculpey to make two matching balls for the eyeballs. The advantage of using Ultralight Sculpey over other brands is that it's lightweight enough to float after baking. However, it's also very soft and can be tricky to work with. I made one eyeball with half Premo, half Ultralight, and it floated just fine — so try experimenting with different mixtures.

A note on floating: My first batch of eyeballs floated iris-side up — which is not ideal for viewing from the jar's side. After some further test, I found that whichever way I baked them is how they tended to float. So you should be fine if you bake yours as shown in the photos. If not, another trick is to add a weight (such as a core made from a heavier clay) towards to bottom to help it float right.

Cut out irises. Use a pasta machine to roll out a very thin sheet of whatever color clay you want your irises to be, then cut two matching circles using a small round cookie cutter. If you don't have a pasta machine, use an acrylic brayer, jar, or clay-dedicated rolling pin. A small circle template will work in place of the cookie cutter.

Optional: Irises are rarely a solid color. If you're going for a more realistic look, try adding gradiations and specks of color. One way to do this is to dab the piece with alcohol inks before baking. A Skinner blend would also work.