I made this proyect like this (but with diferent design):
http://www.cutoutandkeep.net/projects/black_widow_fascinator
you need principally:
-a base in shape of helmet, that you can cut as you want.( I bought this in a costum shop)
-black plastic headband
-different feathers
-rigid net tulle
-rigid tulle for base (in this fascinator)
-fabric tulle (to cover the base)
- a brooch ( that I bought already done)but I sticked a plastic halloween spider in this brooch

I sewed the complements to the base and them sticked the base to a headband.