To propagate Zinnias, the most common method is by taking stem cuttings from a healthy plant, removing the bottom leaves, and placing the cut end in water or directly into potting mix until roots develop; you can also propagate Zinnias by collecting and sowing seeds from mature flowers. 

Key steps for propagating Zinnias from cuttings:

Select a healthy stem: Choose a non-flowering stem with several sets of leaves from a healthy Zinnia plant. 

Take the cutting: Using sharp shears, cut a 4-6 inch section just below a leaf node. 

Remove lower leaves: Remove the bottom leaves from the cutting, leaving only a few leaves at the top. 

Optional: Dip in rooting hormone: To encourage root growth, dip the cut end in rooting hormone. 

Root in water: Place the cutting in a container with fresh water, ensuring no leaves are submerged. Change the water regularly. 

Plant in soil: Once roots develop (usually within a couple of weeks), plant the cutting in a pot with well-draining potting mix. 

Propagating Zinnias from seeds:

Collect seeds: When Zinnia flowers fade, allow the seed heads to dry on the plant, then harvest the seeds by gently rubbing the dried flower head to release them. 

Store seeds: Keep seeds in a cool, dry place until ready to plant. 

Sow seeds: Plant seeds directly in the garden soil after the last frost, or start them indoors in seed trays several weeks earlier. 

Important considerations:

Timing: The best time to take Zinnia cuttings is during the early to mid-summer when the plant is actively growing. 

Light requirements: Ensure your propagated Zinnia plants receive plenty of sunlight for optimal growth. 

Soil conditions: Use a well-draining potting mix when planting cuttings in pots.