There are numerous you tube videos, check them out then give it a try.
In one they use a sandwich baggie and fill with moist potting mix. They slit
the bag and place this around the branch. This worked the easiest for me.
It was harder for me to take a handful of damp sphagnum moss and wrap
that around the branch. I tried to do four on an Alano sapodilla and four
on a longan. I offered one to the person how owns the longan and he said
sure. I tried to do it a couple different ways to learn and see which way worked
the fastest. The tree will split where you attempt to do this and if you ever notice
the trees that nurseries use the budwood for grafting or airlayers are always bushy
afterwards and eventually will produce more fruit. It doesn't hurt the tree. This is a
great time to do it. The aluminum foil is to keep the sun from drying out the medium
and also to keep excess moisture out. After the airlayer roots and you place in a pot
then keep in indirect light for a couple months for the airlayer to root completely,