Cut off the hood from an old jumper or hoodie (mine was very lightweight, a cardigan might work well so experiment.
Stitch a hem where the hood was cut so that it doesn't unravel.
Add ribbon so that you are able to tie your bonnet.
Decorate with lace, buttons, beads or anything that takes your fancy. I decided to go for a rough and ready victorian urchin look but as said above, experiment!
Enjoy your bonnet. Very good for cosplay, LARP, dressing up or keeping your ears warm on a frosty autumn morining.