Source: Bonsai Hunk Blog.
The easiest way to propagate most Ficus trees is to use cuttings. Cuttings of any size Ficus generally will root. However, some people prefer to use an air-layering process as this can be a safer way to propagate a specific piece.
Ficus salicaria that should be shortened
A groove is created around the area in which roots are desired
A plastic bag with sphagnum moss is wrapped around the grooved area- water the moss to keep it moist
Once roots fill the bag the air-layer is cut off
The top of the tree with all the moss is placed into a pot and watered normally. The lower section of the tree will sprout out in 1-3 months.