In my research into making a top hat I found several very useful instructional sites, which I will share here, but because of the style I wanted I couldn't actually follow any of them to the letter and had to create my own pattern.Leather Learn: Top Hat
I believe this site uses a pattern that you can buy at Tandy Leather. The site does have a copy you can download and resize.Tom Banwell: Leather Hat Tutorial
This one has two different patterns that you could potentially use.
I printed off and put together all three patterns as well as one I free handed. Then I tried to combine aspects that I wanted into one SUPER PATTERN! Okay, maybe that's a little much but, regardless, this is what it looked like.
It was at this point I realized it looked like I was making a pattern for underwear...
Now that you can see what I came up with let me explain why the pattern looks the way it does. The three parts of the pattern are the Crown, the Brim, and the Top. The wavy bottom line on the crown is because I want the brim to swoop down in the front and back, but curve up on the sides. The sides of the crown are angled in towards the top because I didn't want it to flare out like most top hats. Oddly enough the top of the crown has to be curved so it will be flat when it's rolled into a cylinder. If the top goes straight across it actually ends up with an odd peak in the front once you put it together. This is caused by the angled sides. If they were straight up and down you could cut the top straight across.
As you make your pattern you'll want to check to make sure the fit is right. I have a felt top hat that my wife bought me for Christmas that is a perfect fit, so I took measurements from that to make sure that this new top hat would be of similar height, brim width, and fit. If you don't have that, it's okay, you can just measure your head and either stick to a pattern or just eyeball how big you want it to be. It's actually not a bad idea to make it slightly bigger than you need it because the leather can shrink. If it turns out a little too big in the end you can just add a sweat band to the inside, which isn't a bad idea anyway.
My measurements were:
Crown Circumference: 24 5/8"
Crown Height: 6"
Brim Width: 2 3/4"
Regardless of how you choose to do it remember to continually check the fit by actually trying it on your head, because that's the best way to know if it's going to fit. Another plus side? You can take goofy pictures of yourself wearing it!