Harvesting radish seeds is a little more labor intensive than some other seeds but still pretty easy to accumulate a decent number of seeds in a short period of time.
If you let your radishes be they will grow little flowers and pods like you see below. I like to let mother nature take care of this process as long as possible pulling them only after temperatures start to dip and heavy rains start coming. At that time I will pick the plants and hang them in the garage to dry a little longer.
Once the pods are dry, they should resemble those of the ones below.
All that is left to do is use your finger nail to split the pod open and with a quick swipe of your finger pop the seeds out into an awaiting container. One other option is to simply leave them in the pod and open them up during planting. Now they will take a significantly more space though if you only need a few seeds, definitely a viable option.