My experience with letting basil self sow, has been that the seeds germinate so qji,kly and easily, that they start growing on the first couple of nice days in the late winter, then the very tender seedlings wont survive the first cold nights that come along.

Ive had more success if I keep a little more control.

Maybe if you are in a mild climate you would have more success letting them self sow.

Let the seed pods turn brown and then harvest them. You should have plenty of seeds. They will come up everywhere if you wait too long. My greenhouse is full of basil seedlings.

Yes the seeds need to turn brown and dry before harvesting. As for when for me here in Texas I have harvested them the following spring as the seeds stay firmly attached to the plant. I have never had them self sow.. The seeds you harvest will be more viable than any you get from a seed company. I plant them as they are without removing the husk, etc. Right now I am waiting for my Thai basil to produce seeds. I let the plant in the soil over our "winter" here and it is putting on new flowers after I harvested seeds last fall. I just wanted to see if it would live as it kept a few green leaves all winter. It does put on beautiful flowers so I am also enjoying the blooms.