I'm not an amazing seamstress so here's what I did, but I'm sure there are better ways. I used wool felt. I bought one yard, which was more than I needed, but I always like to have a little extra just in case. I left it folded in half so everything would be two layers thick so it would be stiff enough to stand up. First, I cut out the top of the hat about twelve inches high plus a seam allowance. It was the shape of a piece of pie about 1/3 of the pie. I decided to decorate my hat with polka dots. If you don't sew, you can make this hat just using glue (since it's wool-felt I would use fabritac glue) so you could glue the dots on, but I decided to sew them all on by stitching stars with contrasting black thread. I think they showed up pretty well.
Then fold the straight edges together with right sides together and sew down the straight edge (doesn't just the stiching look fun? You could do a really fun design just with thread).
Then turn the hat right side out
Next using a chalk pencil or fabric ink pen, trace a large circle for the brim of your hat. My circle was about 14 inches across and then I decided to give my brim a scalloped edge so I traced half a cup around the edge of my circle. Then since you want this to be two layers thick, pu thte layers together and sew along the outside of your brim.
Trim the brim close to your stitching then draw a circle in the middle of your hat for the hole for your head. My hole is about 7 1/2 inches across. Then sew over that circle and cut out the inside fairly close to your stitching.
Slide the brim of your hat over the top (from the top down) until the circle of your brim is tight on the top of your hat. Pin the extra edge of the top section of your hat so it's sticking out below the brim and stitch all the way around. Trim the excess.
I decided to add a toole ruffle around my hat and since I know a few of you don't so I'll explain how to do that too. I cut long strips of toole 5 inches across, then I stacked two strips together and folded them in half so they were two and a half inches across. Then I sewed all the way down the strip with the biggest stitches that my sewing machine can make.
Next begin to pull one thread out, which will gather the fabric. You can pull the top or the bottom thread, but se sure to always pull the same thread. If you switch from top to bottom or vice versa, your stiches will lock and you will no longer be able to gather the fabric. Just keep pulling until you have the length that you want, then spread the gathers out evenly.