Find a piece of cardboard that is approximately A4 size and cut it into a frame. I chose a frame width of 5cm and a cardboard that was around 3mm high. I placed the frame on a thick plain cotton fabric, but a dishwashing towel should work fine too. Afterwards I placed a hand full of seed pods in the frame and crushed them with a rolling pin. The cardboard keeps just enough distance to not harm the seeds. What you have left will be a mix of shells (=chaff) and seeds.
There are different ways to separate seeds and chaff. The fact that the seeds are heavier than the chaff makes it an easy, but not necessarily quick process if done by hand. Winnowing works well, which basically means blowing air through the mix to separate the two. As said, there’s multiple methods to do so, with a grain sieve, with a flat basket (see above) or how I did it: sit outside with slight wind and let the mix fall gently in a distance from one bowl to another. The wind will carry away the chaff. This procedure needs to be done many times to have everything fully separated.
Attribution top right picture: Worfeln by Hans Ole Brasen / CC BY-SA photo by ANKAWÜ