Many herbs can be harvested throughout the year.
For leafy herbs, (parsley, chives, sorrel, etc), cut the stems about an inch above ground level. Trim & discard any discoloured or damaged leaves.
For shrubby herbs, (rosemary, sage, thyme, etc), cut the growing tip of the plant. Do not cut into the old, woody stems. Always make sure there is new growth visible below the cut.
Most plants will benefit from being harvested regularly. This encourages vigorous, new growth & prolongs the supply of fresh material. It also improves the shape of the plant, making it bushy & sturdy.