Certified GKH Gardening Expert
Trimming cilantro is just like giving it a haircut. Cut straight across the stems about 1/3 - 1/2 the way down the plant. New leaves will grow from the base of the plant.
You can start to harvest cilantro as soon as you have a few stems on the plant, though this is not likely to be enough to make a dish with. So, you can wait to harvest until you feel there is enough to cook with. Don't wait too long or be too shy about harvesting. Harvesting can help keep bolting from occurring, which ruins the flavor of the plant.