I recently made this groom's cake for a cowboy who was also a bull rider in the rodeo. Thought I'd share how the hat was done:
Cover the underside of the hat with plastic wrap.
I used glue dots to help hold the plastic wrap in place.
I forgot to take a picture... but the next step is to use the pattern and cut a circle out of fondant for the brim. The fondant used for the brim needs to be a little stronger. You can make it stronger by kneading gum paste or a little tylose together with it. (Using a 50/50 mixture of gum paste + fondant not only strengthens it, but it also shortens the drying time.) Place the fondant circle over the plastic covered brim of the real hat and let it dry for 5 to 7 days. The fondant needs to be completely dry before you remove it or the brim might crack and break. Once it's dry, put the cake on top of the brim and decorate the hat as desired.