After running around in the pouring rain last weekend without a hat, I decided to finally make one. I created this whole pattern myself, so hopefully it will work well for you. I will explain how to make your own pattern, using your own measurements, so it will fit you perfectly.
I rated this as tricky, although if you are a beginner and ready for a challenge don't be intimidated. Hopefully I explained this well; if you have problems just ask in the comments or message me.
Also I highly recommend using a pair of old trousers for fabric, because denim can be very expensive and I picked up the trousers I used for a 2 bucks at a thrift store.
Circumference-The edge of a circle
Diameter - The length of a line running through the middle of point of a circle to the edge.
Take the measurement of the circumference of your head about an inch above your eyebrow (you may want to get somebody to help you with this). Now divide that measurement in half and add one inch (this is for seam allowance).
Create a rectangle on your paper with the length of your new measurement and five inches wide. Fold your fabric in half with right sides together and cut this out through both sides so you have two pieces.
(If you are using trousers, you don't need to fold, just cut right through both sides of the leg)
Now we are going to create a pattern for the top of the hat and this is going to require some math, but don't freak out it is pretty simple.
Divide the circumference of your head by Pi (3.14) to get the diameter. Now add one inch to the diameter and times that by Pi to get your new circumference. (Basically you just added your seam allowance)
Now draw a circle with that circumference using a compass or something circular from around your house (I used a bowl). Cut one circle out of your fabric.
Sew the lining/interface together as you did with the fabric above, except do not press the seems. Once that is done put the lining in your cap (don't sew anything yet) or iron on your interfacing. Remember not to iron anything from now on if you are using a lining that has plastic or wax in it (the table cloth I used did).
Then take the pattern for the top of your hat to get the appropriate sized arch. Also add a seam allowance around the edge of the overall visor pattern. I just eyed mine, but you can measure out about 1 centimeter around it if you would like.
I uploaded a scan of the visor pattern I used; it probably won't be the right size but it may help you understand. You can find it here: http://i3.photobucket.com/albums/y63/mynameisKat/IMG_0001.jpg
Cut the visor pattern out of your folded fabric (remember right sides together) so you have two pieces.
I forgot to take a picture of this step.
Now cut off the seam allowance edge on the pattern so you have the exact size of the visor and cut this out of your plastic. Pop bottles, ice cream containers, plastic container lids, all work great for this, but if you can't find a piece of plastic then just use a thin piece of cardboard or cardpaper and coat it with duct tape to make it water proof.
Using a piece of plastic with a bend already in it, will work the best.
Hold the top side of the visor against the right side of the cap and pin the visor an equal distant from the side seams along the bottom edge. Now sew it with a straight stitch.
I had to re-sew the visor on four times before I got it right, it is important that the visor isn't lopsided and the bottom seam of the cap is an equal distance from the visor top. Also make sure you sew as close as you can (if not directly on) the straight stitch you did long the inner arch of the visor, this will help it arch and keep it from flopping around.
Cut off loose threads, and trim it all up. Also feel free to decorate it with patches, fancy stitches, writing, chains, pins, etc... I'm going to decorate mine, but I just wanted to create a tutorial for a good base.
Hope I didn't make this too difficult, and I would love to see your versions.