For sunflowers- Are they black oil or confectionery? (doesn't make a difference- I am just curious)
You need to put a bag over there heads to keep your harvest, otherwise the birds will get most of them.
I would stop watering them and either tie a cheesecloth type bag on their heads (they still need air to circulate).
When they are ready, snip them and take the heads (in the bags) into your work area. They need to dry some more- store in cool, dark place for a few days (I leave them all in the bags).
In 3-5 days take them out of the bags and bang the heads together over a clean bucket. The seeds should be lose enough to just come out.
Let me know how it goes
I haven't grown them for years.