The bonnet should sit on the back of your head with the brim framing your face.

For a great and easy period hair style:

Part your hair down the center.
Now make a part from ear to ear.
Braid the two pieces of hair in the front to make two small braids on either side of your head.
Put the remaining hair in a low bun on the nape of your neck or in a snood.
Loop the two braids back and secure them to the bun (if long enough).

You will see this easy and very period hairstyle often in contemporary photographs.

That's right. If the bun is low enough, the bonnet back cutout portion kind of sits on the bun. Hatpins work well if you can somehow get them through the bun.

This site has a good side view of a coal shuttle sort of bonnet frame sitting on a low bun:
www.victorianbonnets.com/1850'...ts.htm