Cut cone shapes from the poster board, as shown in the diagram below.
Roll the cone shapes into cones and staple and/or tape them closed.
At the Party:
Have the children color and decorate their cone hats with glitter glue, gel pens, scraps of fabric or ribbon, and other items.
Glue wide strips of tulle, other sheer material, or ribbon to the peak of each hat so that it cascades down. Also, if desired, you can add elastic chin straps.