Hi Suzie,
Castor oil apparently brings light/energy to whatever organ you are treating. The skin will absorb it and then it will move to the lymph and create movement and dislodging and loosening any congestion, including tumours apparently. It will stimulate the liver to release bile and is good for constipation. You can put a pack anywhere, eg on your head, genitals, though over liver and abdomen, or boobs to pubes as we say!!! :), is the most common area. Urine packs are better over the kidneys though!! :)
If you are quite ill/toxic best to start off doing for just one hour before bed (as though toxins will come out through the skin they will also be released into the bloodstream) then doing an enema in the morning. It's common for people to leave on for several hours or overnight, though I've never tried that.
Hope that helps!
Anne