Air layering is a propagation method for woody plants that allows you to root branches while still attached to the parent plant. It is useful for plants that are hard to propagate by cuttings or if you want your new plant to have a larger size than could be accomplished by taking cuttings.

I've used this method successfully for
- Weeping Fig (Ficus benjamina)
- Long-Leaf Fig (Ficus binnendijkii)
- India Rubber Tree (Ficus elastica)
- Fiddle-Leaf Fig (Ficus lyrata)
- Umbrella Tree (Schefflera arboricola)
- Elephant Yucca (Yucca elephantipes)
- Cornstalk Dracaena (Dracaena fragrans - pictured below)
It should work for most, if not all, woody houseplants, and even for outdoor plants.

Step 1: Tools and Materials

- the plant you want to propagate
- a sharp knife
- Sphagnum moss (should be available at garden centres)
- a piece of (preferably transparent) plastic foil (an old plastic bag will do fine) (about 30x30 cm)
- string
- a piece of thin, hard plastic (I used a piece of a plant tag, a piece of a plastic bottle or yoghurt tub or something similar would work just as well. Basically, you just need something thin that will not decompose if it's damp for a couple of weeks). It should be just a bit longer than the width of the branch you want to root.
- scissors for cutting string and plastic foil
- water
- a bowl to soak the moss in

A couple of weeks later, you will also need:
- a flower pot
- potting mix
- maybe a stake and more string.

Step 2: Preparations

- soak the moss, then squeeze out the excess water. You need enough to make a fist-sized ball if you lightly press it together.
- cut the plastic foil/bag and hard plastic to the proper sizes (about 30x30 cm and slightly larger than the width of your branch, respectively).
- cut two pieces of string, long enough that you can wrap it around your branch a couple of times and easily tie it, about 20-25 cm)
- choose the branch you want to root, and the place where you will cut it (preferably just under a node - that's the place where a leaf is/was attached). If necessary, remove some leaves - you'll need about 10-20 cm of leaf-less branch.

Step 3: Cut the Branch

Make an upwards-slanted cut about halfway to two-thirds through the branch. Take care not to cut so far that the branch breaks.
The way I've learned it, you support the branch with your thumb, then pull the knife towards it with your fingers (as seen in the picture). Place your thumb so that the knife blade would, if you cut all the way through, move past it rather than directly towards it - that way, if you accidentally cut too far, you minimize the danger of cutting yourself.

Step 4: Insert the Plastic

Take the small piece of plastic and insert it into the cut. This will keep the cut from closing up again, encouraging the plant to grow roots instead.

Step 5: Wrap With Moss

Take your damp moss and wrap it around the cut. Like I said in step 2, it should make a fist-sized ball, all around the cut. Don't press it together too tightly, it should feel... how to describe it? ... sponge-like.
Given time, the plant - encouraged by the moisture and the cut - will grow roots into this moss.

Step 6: Wrap With Plastic Foil

Holding the moss in place with one hand, wrap the plastic foil around it. It might be good to have a helper at this stage - this is a bit tricky the first couple of times.
Tie the plastic tightly below and above the ball of moss.
If necessary, tie the branch to a stake to support it (being halfway to two-thirds cut, it may be in danger of breaking).

Step 7: Cut the Rooted Branch

After a couple of weeks or months (depending on the plant and its growing conditions), you should see roots growing through the moss. This is why I prefer transparent plastic to wrap around the moss - makes it easier to check the progress.
If it takes longer than a few weeks, you might want to open the plastic foil, and make sure the moss is still damp. Otherwise, just leave it alone and wait.
When you can see well-develloped roots, cut the branch below the moss ball.

Step 8: Potting Up

Remove the plastic wrap, but leave the moss ball alone so as not to damage the roots. Pot the new plant up using good-quality potting mix and a smallish pot - I prefer not to go over 15 cm diameter for that first pot, smaller than that if the plant is little enough to stay upright in a smaller pot.
The reason behind this is that if the pot is a lot bigger than the still-small rootball, the soil will stay wet for a long time, because the plant can not yet take up so much water, and this can cause the roots to rot.
So you should also take care with watering during the first few weeks, keeping the soil moist enough that the plant doesn't wilt, but never, ever truly wet.