Seeds need to be fully ripe before they can be used for propagating. That means, when they're brown and falling from the plant. Some pix at the link below.

The Âseeds are technically the fruit, each containing 2 true seeds. Germination can be improved by rubbing the fruits until the 2 seeds separate. Soak the seeks for 3-4 days, changing the water twice daily, then dry the seeds for 8 hours before planting. Sow seeds about 5mm deep.

To harvest seeds, let the plants grow until the first set of seeds begins to turn brown and dries enough to crack when pinched. At this time, cut and hang the plants to dry over a catch-cloth.

Here is a link that might be useful: coriander seed

Thank you! Now I can get started.. Laura

As Daisyduckworth has pointed out the seeds need be separated which is same as scarification of seeds.For better germination place the seeds between folds of bath towel or similar cloth and crush the seeds gently tapping by a wooden piece. This will separate the seeds into two halves as well as break the seed coat to remove the water barrier helping seeds to germinate faster.

I am not sure if it is just the climate we have hear in the NW but all I have to do is let one plant go to seed, leave it there until late fall when I clean up the beds, and I have a billion little cilantro plants the next spring. I have only once 4 years ago planted cilantro, now I have it coming out of my ears. Even when I have tilled the beds, enough seem to make it to the surface to germinate. I just love volunteers, no effort.

I have to agree with Katj75. Coriander usually self-seeds readily without any human intervention. But it's good to know the whole story! Too many people will collect the unripe seed and expect them to grow, however. I prefer to let Nature get on with doing what it does so well.