When the seed head is fully ripe, you will see tiny perforations covering the top of the disc. These holes allow the mature seeds to escape and scatter into the environment.
Removing Black Eyed Susan seeds from a mature seed pod is very easy. Often turning the pod upside down over a container and tapping it will dislodge the seeds, and they will fall out.
Separate the seeds from the chaff. This process can take a bit of effort, especially if there is a large amount of plant debris mixed in with the seeds.