The ring core that you crochet around is brightly colored self-stick felt or foam. For the tambourine, you sandwich a metal ring between the felt/foam layers (and also string on jingle bells). The book provides a step-by-step photo tutorial so it’s easy-peasy.
As the designer, I’m very pleased with how the pattern is published in this book. I’m also inspired by the possibilities of designing with the self-stick felt and foam sheets. As the crocheter, I’m very happy to say that I enjoy crocheting this project and seeing kids enjoy playing with it!