Hi,
Once the flower head have dried completely, cut them off whole into a bag. Bring them inside and crumble onto a plate or paper. You should find seeds falling out as you crumble. They are small ovals. The variety I have has light brown seeds, but seed color could vary with variety.
I found a pic from Wintersown. Check out the link.
Remy

Here is a link that might be useful:

thanks!