Once the flowers start to die, you need to bag a few blossoms. Organza gift bags work nicely. Make sure the bags are tightly secured around the stems. Cut off all the other blossoms. Once the seed ripen, you will see them fall off in to the bag, and you'll know it is time to cut the stems.
Remy

remy: You have such good advice on so many topics! (Pam, new visitor, usually at Wintersowing)

Thanks Pam!