Cut two leaf shapes out of green felt, and wrap the bottom of each leaf around a 4-inch piece of craft wire. Use hot glue to secure the felt around the wires. When the glue is dry, twist the wires together and bend the exposed ends upward to create a loop.
To make the felt acorns, you'll need a set of real acorns. Collect these in your backyard, or look for them in a local park; you'll need as many (or more) as the number of napkin rings you plan to make. Separate the acorn tops from the bottoms. Use hot glue to attach acorn tops to pom-poms.
To create the finished napkin ring, use hot glue to secure the wire loop to a plain wood napkin ring. Before the glue dries, add the pom-pom acorn over the wire. When the glue is dry, adjust the wires to position the leaves as desired, and place around a pretty cloth napkin.
Bonus idea: Instead of gluling the felt acorns to a napkin ring, use them to decorate a modern wreath. Use floral wire or hot glue to attach the acorns to a plain metal ring for a gorgeous fall look.