Air layering is a propagation technique that involves creating a rooting zone on a healthy, mature branch or stem of a houseplant, while it is still attached to the parent plant. This method works well for plants that have a woody or semi-woody stem, such as ficus, philodendron, and rubber plants. Here are the detailed steps to propagate a houseplant with air layering:

1. Choose a healthy stem: Select a healthy stem from the parent plant that is at least 1/2 inch in diameter, and has no damage or diseases. Choose a stem that is at least 8-12 inches long, and is growing vigorously.

2. Identify a spot for air layering: Identify a spot on the stem where you want to create a new plant. Make sure that the spot is at least 6-8 inches from the tip of the stem and has several leaves attached to it.

3. Make a cut: Make a 1/4 inch deep cut around the stem, just below the spot where you want to create the new plant. Make a second cut about an inch below the first cut, removing the bark and exposing the cambium layer of the stem.

4. Apply rooting hormone: Apply a small amount of rooting hormone to the exposed cambium layer of the stem. This will help to stimulate root growth.

5. Wrap with moist sphagnum moss: Wrap the exposed cambium layer with a ball of moist sphagnum moss, making sure that it covers the cut area. Cover the moss ball with plastic wrap or aluminum foil to keep it in place.

6. Secure the wrap: Secure the plastic wrap or aluminum foil around the moss ball with tape or twist ties. Make sure that the wrap is snug and secure to prevent moisture from escaping.

7. Wait for roots to grow: Roots should begin to grow in 4-8 weeks. Check the moss ball periodically to make sure it stays moist. Do not water the parent plant during this time to prevent the moss ball from drying out.

8. Cut the stem: Once roots have developed, cut the stem just below the moss ball, using a clean, sharp knife. Remove the plastic wrap or aluminum foil, and carefully remove the moss ball.

9. Plant the new plant: Plant the new plant in a small pot filled with well-draining soil. Keep the soil moist, but not waterlogged. Place the new plant in a bright, warm location, but out of direct sunlight.

10. Care for the new plant: Water the new plant regularly, and provide it with the appropriate light and humidity levels for its species. With proper care, your new plant should grow and thrive!

Air layering is a great way to propagate a houseplant without having to remove it from the parent plant. This method can be particularly effective for plants that have a woody or semi-woody stem, such as ficus, philodendron, and rubber plants. With a little patience and care, you can enjoy the satisfaction of watching your air-layered cutting grow and develop into a healthy, thriving plant.