To summarize, States can agree on international law begin made in any way they wish. Once they agree on a method, the matter is over. As I have pointed out, every few y ears we invent a new method; there is no end to ingenuity of human beings. by the year 2000, there might be one or two more methods. We are still applying the 19th century rule that international law is made by the community of states, but in every generation the community has been able to invent new methods for crystallizing international law. We finally have accepted the principle that it is important to be able to establish new international law quickly in certain circumstances. We cannot wait any longer for the ratification by all states because it takes too long. We read recently that the Security Council adopted a resolution establishing the International Tribunal for War Crimes in Bosnia. The states agreed: we want it done, it cannot be done quickly otherwise, we don't need a treaty, a decision of the Security Council is sufficient. The tribunal's statute was drafted by the Secretary-General, the Security Council approved it, the tribunal was established. Thus, it was done, promptly and efficiently. An important international institution was established by another new method.
Louis B. Sohn,
Sources of International Law
Available at: https://digitalcommons.law.uga.edu/fac_artchop/192