To collect Plumeria seeds after pollination, wait for the seed pod to mature and turn green or brown. Once the seed pod starts to split open, gently collect the seeds from the pod. Remove any debris or excess material from the seeds, and allow them to dry in a cool, dry location for a few days. Store the dried seeds in a cool, dark place or an airtight container until you are ready to plant them. Properly collected and stored Plumeria seeds can remain viable for several months to a year.