Marigolds can be propagated by seed or by rooting stem cuttings.
From seed
Marigold seeds germinate easily and produce strong plants.
Fresh seeds have a germination rate of about 90–95%. Marigold seeds can remain viable for about 1–2 years. Marigolds are self-seeding, so their seeds will drop and spread throughout your garden. To limit self-seeding, you can deadhead before the blooms go to seed.
From stem cuttings
Choose a healthy stem that is free of disease and doesn’t have flowers or buds
Cut a 4–6 inch length of stem Remove the lower leaves Dip the end of the stem in rooting hormone Plant the stem in a pot filled with a mix of potting soil, sand, and perlite Moisten the potting mix and place the pot in a plastic bag to create a mini greenhouse Put the pot in a warm, bright area, but out of direct sunlight Lightly moisten the potting mix every four or five days When the stem has rooted, transplant it into a larger pot