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In the spring or early summer, start looking for plants that have plenty of new growth. You'll want to take cuttings from these plants because there will be a few different stems for you to pick from. New growth is generally easier to propagate than older growth because it has more “growing” cells that are reproducing rapidly. If you're just starting your garden and don't have any stock plants, visit a local nursery and ask if you'd be able to take a few cuttings of larger plants. Most of the time they'll let you if you take a small amount.  If you have friends, family, or neighbors growing the types of plants you want to propagate, you can always ask them for cuttings, too. Be very careful when transporting your cuttings. Place them in a plastic bag with a few drops of water or wrap the ends in a damp paper towel. Then put the bag in a box to prevent any bumping or crushing of the nodes. The plant should be large and healthy, so that cutting off a small amount won't cause it to die. It should also be free from pests, and non-flowering during the time of year that you're taking your cuttings. It's best to gather cuttings earlier in the morning when the plants are full of water. It will make it easier to cut and they'll be more likely to maintain their water when propagated.

Summary:
Survey your garden for potential stock or “mother” plants. Visit a nursery if you don't currently have a garden. Select a stock plant to cut from.