Air layering is just a simple process of propagation and is probably the easiest method of propagation for beginners. Fig trees root readily during the growing season and after following a few steps, this is pretty difficult to mess up. You can boil air layering down to wrapping a branch with moist soil and after 2 months you'll see root development. Sever the layer below the roots for a whole new tree.
In this video I give a whole breakdown on the process and in the first minute I show the process of a method called the sandwich bag method. It's what I've learned to be the easiest and quickest method of air layering. Feel free to give it a watch:
Some of the most important considerations are the soil, girdling, temperature, what container to use and placement.
I highly recommend a medium of either peat moss or coco coir. Or at least your mix should be made up of mostly those materials. They create very good contact with the branch to give the tree the humidity it needs for root formation. Root formation is really the name of the game and an often critical and overlooked point about root formation is the temperature. When putting on our air layers, we should be aiming for a soil temperature that's consistently around 78F for our chances of root formation to be high. Take the average of the nighttime low and daytime high, which can be used as a rough estimate to determine if now is the right time to put on your air layer. When it's too cold, root formation can be difficult. Roots can grow at 60F for example, even 50F, but formation of roots is a bit harder at those low temperatures. Putting an air layer on in October here in the Philadelphia area usually leads to not enough heat or time left in the season. The same can be true in the summer. If temperatures are over 78F, the fig tree's metabolic rate is slowed somewhat. This is why I recommend that you wrap your air layers with tin foil if they're not in the shade and you are using something clear as your container. You can usually start 45 days after your last frost, but the last air layers should go on at least 2-3 months before your first frost.
This whole process usually takes about 2 months. About 2-4 to see visible roots and potentially another 2 months to see a large enough root mass to feel confident about removal. Once the air layer is removed, the tree has to fend for itself. No longer can the Mother tree provide water or nutrients. Shock is likely to occur, so up potting very carefully not to damage any roots, watering the soil well, removal of lower leaves can alleviate some shock and placing the new air layer in the shade will alleviate some shock.
Lets talk about girdling. For most species that are normally propagated by air layer, it is recommended to girdle them and even apply rooting hormone. In the fig tree's case, I've observed that you can put an air layer on without girdling. The process will just typically take longer, but if we place the air layer on lesser lignified wood, I am seeing no loss in how long the process takes when girdling is avoided. This does come with a caveat though. If the growth is too soft, the moisture from the soil in the air layer can rot the growth. If you place the air layer on growth that's more lignified, well then the branch should probably have been girdled and will likely take longer to develop roots. You've got to find that happy medium.
Lastly, I just want to note that I've waited until I saw my first 1-3 light frosts before taking off some air layers. So there really is no rush to remove them. This allows the tree and the air layer to start to enter dormancy and this means that the shock that I mentioned above is a lot less to deal with. This is typically when I've removed them the last few years and in this last season, I ran out of time and removed them the following spring. Most died due to winter cold, but some survived and are now healthy trees.