I'd try putting the flower head in large containers without lids, to dry out. When the flowers are dried out, shake the heads to free the seeds and collect the seeds in paper bags. Label bags and store in the fridge. Other than that, you could do the same as herbs. Tie the plants together and hang them upside down over a sheet or newspapers( can be done inside) and wait till the flowers are completely dried. Collect the seeds the same as above.