to be honest i dont know the answer to your question either, but if i was going to do it id leave it until all of the pettles had fallen off and let it dry on the plant, however the birds love them so you could net them or chop the head off and hang it up somewhere to dry, once dry pick out the seeds.
like i said thats only a guess on how you do it, im sure someone with more knowledge will be along in a bit.
thanks
matt