Keep an eye on the cilantro plants after they flower, usually in late summer or fall. Shortly after blooms form and pollinate, small, brown circular seeds form at the tips of the plants. Once the seeds begin to dry and drop on their own, harvest them.
Lay old newspaper around the bottom of the plants to catch any fallen seeds. Cut the stems of the coriander plants an inch or two above the soil. Place each plant upside-down in an empty paper bag. Dump any seeds that fell on the newspaper in the bag along with the plants. Handle the plants carefully when cutting them to reduce the amount of fallen seeds.
Place the bag in a warm, dry area such as a closet, kitchen pantry or any other area of the home that doesn't get regular exposure to moisture. Leave the bag in the chosen area for two weeks, or until the plants feel thoroughly dry and brittle. Once dry, grab the top of the bag and shake it vigorously for 15 seconds to separate the seeds from the stalks. Pull out the stalks and discard them.
Pour the seeds in an envelope. Mark the envelope with the contents as well as the current date. Store the envelope in a dry, dark area until they are ready to be used. If planning to use the seeds to grow more cilantro the following year, they can be stored in the freezer to make them completely dormant.