The Oscola Guide contains information (p. 37) how to cite Judgments of the European Court of Human Rights (which is missing in the Oscola Citing International Law Guide).
Textbooks: Consult a textbook when you start with a specific topic that is still (partly) unknown to you. When reading textbooks etc. note down document references. A document reference is a good starting point to find literature.
Journal Articles: Journal articles will point to relevant sources law, key concepts and legal developments. These journal articles contain furthermore: book reviews, bibliographic information, reports of cases. Yearbooks are useful for orientation, quick reference and the development in a specific field of law.
Commentaries: Commentaries on different treaties are also very interesting sources for information, references etc.
If documents of the Max Planck Encyclopedia of Public International Law (MPEPIL) do not open in your nestor course , go to the database part of this website. Search for the document directly in MPEPIL and it will open.