Wait till the pod turns brown. As soon as it is brown pick it but be careful as they might pop open as you pick them. Let them dry inside, again be careful as when they are ready they will pop open so make sure they are covered or you will have seeds all over the place. Of course if you have a patio anywhere within 70 ft of the plants you can just wait for them to pop on their own (sounds like it's sleeting outside) and then just sweep them up off the patio.

If you are trading the seeds please make sure everyone knows that they are very invasive and each plant will make hundreds of new plants.

JP