I've never designed my own hat before...what i usually did was modifying the pattern...yet, it was difficult, too...one day, i learned much about crocheting and i love crocheting hat!...i use japanese books most often and the diagram helps me a lot...
When i tried to design my knitted hat, it was just like a plot in my head that i'd already imagine the diagram...and i did make the stitches diagram...i also made the shape pattern, too, by using the "diagram" in my head...i used japanese crochet pattern technique...it worked!...
May be it's not the best knitted hat in the world but it's my original knitted hat...(^.^)...
http://www.flickr.com/photos/sweetnichan