I got the inspiration to do this from a good friend which made her hat herself, too :) but actually this tutorial is my idea, don't know the way she did hers.
First, get an old roll of toilet paper and estimate the size of the brim (don't know wether that's the right word...), well, the thing thats on the bottom of your hat.
Draw it on a piece of stable cardboard, eg. from an old drawingpad.
Don't forget to also cut out the top of your head with just copying the size of the upper hole on another piece of cardboard and cutting it out.
At next, glue everthing together with a hot glue gun. (Thank GOD for them!!!) It doesn't have to be very clean, you'll cover it with the fabric anyway.
Once you have the shape, I think it's pretty easy to cut the wanted fabric into the matching pieces. Do one for the bottom part, the round part in the middle and at last a simple circle for the top.
Now the decorating part. (yay)
Glue the ribbon (mine is satin) around the hat, you can also add some lace, small flowers, or like me a old short chain with pearls (mine was from an old bracelet - that's the reason I always store those things :D)
Now you can glue your hat on a hairband or add a hairclip to it, something that keeps it holding in your hair ^^