The basis of this is a mould made from artists' plaster-bandage, roughly the size and shape of the top of my head, smoothed down a bit with sandpaper & DIY filler and painted to seal the surface.

Then I used this to shape the crown - it was just a question of damping the sinamay and stretching it and smoothing it over the mould. If you have a proper hatblock you can pin it, but being a cheapskate I had to use a combination of tucking the edges under and securing with a big elastic band. Then I dried it with a hair-dryer.

The vertical edging was simple - I just cut a strip of sinamay a bit more than twice the width I needed and a little longer, folded down the rough edges about 1/2" and then folded them again into the middle, and wrapped it round the crown. I made a feature of covering the join at the back with a narrower band of sinamay, folded as before and wrapped round vertically.
Then I decorated it with merry widow veiling and a feather mount from Randall Ribbons and a hairclip I picked up from, um, somewhere or other.