Let some basil plants flower and go to seed
Wait until the flower spikes turn brown and dry
Cut the seed heads from the plant
Place the seed heads in a paper bag or on a dry tray
Let them dry fully for several days
Rub or crush the dry seed heads gently to release the seeds
Separate the seeds from the chaff
Check that the seeds are dark and hard
Store the seeds in a labeled, airtight container
Keep the container in a cool, dry place