UK CAA used to publish a CAP (civil aviation publication) booklet which gave some basic underslinging info for starters. Can't remember its CAP number.
If you are going to undertake underslinging I STRONGLY recommend getting some proper training. Underslinging can vary from "simple" point to point underslinging of compact dense loads which may (or may not) fly relatively easily, to complex underlinging of construction materials which may involve precision positioning - electricity pylon construction in mountainous areas, offshore flaretip replacement, "live" inspection of electricity high voltage cables with people in baskets slung under the helicopter etc.
As to short line verses long line - depends on the job.
If not done properly, this is very dangerous work which will kill you the pilot, or people on the ground, with no advance warning.