Glue the template to card. Allow to dry and measure your head with tape measure. Cut out the size closest to your head size, start smaller and cut larger, until you can fit to your your head, just above the ears. Keep exactly on the lines – it may not be 100% touching all way around yet.
The patterns includes an allowance for the width of buckram and materials, to a point! If you are using a thicker fabric than lining, such as velvet on under brim be prepared to make the head oval opening larger and hat sides to match.