I break the the seed heads with my hands, then shake the seeds violently to break off the stems. Then I use a couple of screens (one w/holes slightly larger than the seeds, followed by one w/holes slightly smaller) to remove the majority of soil, dust, and chaff. Whats left can be used as is, but you can carefully winnow away some of the stems by pouring the seed into a large bowl, under a light breeze. A little seed might be lost too - so I do this over the lawn, where it is unlikely to cause future problems.