Help: Doll's Head-piece

[TheCDFreak]

For my next cosplay I am doing Doll from Black Butler/Kuroshitsuji. I am having a difficult time with her head-piece/hat. I was wondering if anyone would know how to get the volume of the piece before adding the flowers. Also I was hoping for it to be somewhat shaped to my head, so if you have any advice on that to would greatly appreciated. Any advice at all and suggestions on material would be the greatest thing ever.

Thank You
-Gabby