When using shrink plastic (also called shrinky-dinks), it is important to remember when to use the rough side of the plastic and when to use the smooth side. Most projects don't have you color on the smooth side of the plastic; however, to achieve the pop that gives this necklace its unique look, the coloring is done on the rough side, and the outlining is done on the smooth side. Remember that the color intensifies during the shrinking process, so don't use colors that are too bright when you're coloring, unless you want that look.
Plan an owl design on paper. Make sure it consists of several areas that can be cut into separate pieces and has some detail. See my necklace above for inspiration. Obviously shrink plastic shrinks, so make your design much bigger that you want the necklace to be when finished.
Alternatively, if you don't want to plan a design, you could find a pattern and trace it. I used an owl pattern from "Shrink Plastic Jewelry" from Karen Phillips.
Cut these pieces apart, and trace each one onto the rough side of the shrink plastic using a pencil.
Start heating your oven according to the shrink plastic's package. Color in the outline on the rough side. Make sure that there are no blank patches of plastic and that your coloring is even.
Flip the plastic over and outline the shapes with the black sharpie on the smooth side of the plastic. If you have a sharpie with two tips, use the thicker tip for major outlines and the thinner tip for detail outlining.
Punch holes for the jump rings on the pieces like I did above. Remember not to punch too close to the edge.
Shrink the plastic according to the package. I found that baking just one piece at a time is helpful. It's okay it a piece folds over while it's shrinking, as it usually does this. However, if it seems like your piece has finished shrinking and is still stuck, pull it out of the oven, quickly pull it apart from itself, and flatten it a little bit.
Attach the pieces together using the first six jump rings. Add one more jump ring at the top, too. String the cord through the top jump ring.
Knot the ends of the cord, close the clamshell cord ends onto the knots, and use the last two jump rings to attach the clasp. You're done now!