When the flowers die off they will be replaced by pods. The lighter one on the right is about to crack open. When that happens the little seeds which are attached to fluff will float away . It is nearly impossible to collect the seeds in that state so you should try to get them when they crack open but haven't gotten to the fluffy stage yet.
They are easy to collect when they look like this. Just run your finger down the side and they will come off the base easily. Slide them into a container. Ignore the white beans in the background.
They will initially be attached to the softest fluff you have ever felt and the goal is to separate the seed from the fluff. I put a few coins in the container and shook it gently. This seemed to work very well. You can easily remove the fluff which collects at the top. The fluff is actually water repellant silk and was once used to insulate moccasins. They were also used to stuff life jackets for the military.