Wait for the blossoms to fall off the hollyhock in the fall. In a blossom's place will be a small, podlike fruit where the seeds are located. When this fruit has dried to a brown, paper-like texture, your seeds are ready to harvest.
Cut the seedpods from the hollyhock with pruning shears. The seeds inside should be black. If they're not, wait a few more days before harvesting. Cut them directly into the plastic container in which you plan to store them. The container should have a lid for storage.
Remove the papery outer covers with your hand, leaving only dark seeds in the container.
Label the container with the name hollyhock as well as the date harvested, using a permanent marker.
Place stored seeds in a cool, dark place until you're ready to plant in the spring.