Step 1: Materials & Pattern
Create the pattern using the dimensions shown on the grid. (The pattern turned out to be too big for the brim. Cut it down to 13 inches wide, then you can fit it from there. I included a metric conversion chart from my grid.)
Step 2: Mark the Pattern & Pin
You can see that the sweater was too small for the entire pattern; use part of the arm, then sew it to the top of the hat. (turn the arm inside-out before you start.)
Turn the material inside out, then draw a line around the pattern.
If you are using a non-stretch material, or one that has a tight weave, you should be able to cut and then sew.
Step 3: Sewing
Turn the arm inside out, cut into a tall triangle, then sew to the top of the hat.
Cut the brim off.
Turn the bulk of the hat right-side out, but leave the brim inside-out.
Sew the brim to the bottom of the hat. The wrong side of the hat should be facing the right side of the brim.