The Sorting Hat that we made for Emily's Harry Potter party is exactly the same hat as the witch hat, except in brown felt, and we made the cone about 3" longer (taller) than the original witch hat, to have extra material to scrunch up and fold over like in the photo. To add this 3", simply extend the height of the cone upward (don't mess with the bottom edge at all) by 3".
Everything else remains the same, including the seams and method of construction.
I made the face in two stages.
In the first stage, I formed the face.
Create two folds. The first is a horizontal fold which forms the base of the eyes. The second is the forehead, which you should sink low in the middle in a V to meet the first horizontal fold. Shape the sides of the eyes as shown to create two hollows.
On the WS of the hat, make small stitches to hold the corners on place, as well as the point of the V. The mouth is a long fold that's higher on one end than the other. To hold this in place, ladder-stitch (on the WS) the mouth shut. The next photo shows the stitches and their locations.
The second stage is creating the scrunched-up folds above the face. There is no formula for this - simply crumple and crease the tip of the hat until it forms a shape you like. Here is ours from different angles to help you.
When you are happy its shape, paint the entire crown with Stiffy or some other liquid that provides stiffness when it dries. Try liquid starch or even a solution of white glue and water. Use as many coats as needed to obtain the desired rigidity.
If you are planning to hide devices in the hat for transmitting a voice, sew a pocket on the WS before you form the face and paint the hat with Stiffy.
The template for the witch hat, from which we made this Sorting Hat, can be purchased in the Wizarding Party Printable pdf file here.