Form the half-circle into a cone. Make the base as wide as you can or wide enough to fit on your head. See details on making a cone hat.
If you don't mind a smaller hat, you can print out this birthday hat template on A4 or Letter size red cardstock. Cut out the template and apply glue on the tab area. Form the template into a cone by bringing the hat's opposite straight edge over the tab area. Press to glue in place.