Take old poster board and cut out a large cone shape with the bottom having a slight curve to it. Then another large circle that will be the brim of the hat.
Fold the cone around into itself so you have a 3D cone shape. Use masking tape to secure. Then use masking tape again to secure the cone to the circle brim.

Next using your favorite paper mache paste, (you can find TONS of reciepes for this online)and lots of strips of paper start to cover your hat to firm it up. I like using old grocery bags because I think the brown paper makes a stronger end product. While it is wet you can bend down the top of the hat and shape the brim however you want to.
Then set it out to dry. Our's took several days to dry competely.

Once it is dry you can paint it black. I found that flat spray paint gave it a really nice finish to work with. Then you can embelish it however you like. I used vintage ribbon, feathers, beads and copper wire. Have fun, I would love to see more versions of this.