To make the miniature chef's hat or toque, find a tube shaped mold roughly the size of your doll's head. Suitable molds include a lipstick tube or the top of a bottle for a 1:12 scale chef's hat, a cork or a piece of dowel for a 1:6 scale hat, and a pen tube or skewer for smaller scales.
Center the fabric or paper circle on the mold and gently bend the outside of the hat over the mold. Hold the paper or fabric so the center of the hat remains at the top of the mold and begin making fine pleats. Make the pleats as small and as evenly spaced as you can. If making pleats is too difficult, you can just gather the edge of the circle to the sides of the mold. There are many styles of chef's hats!
If possible, glue the pleats together at the bottom edge and leave to dry.