For source collection, choose the relevant source page: Treaties, Cases, etc., and locate a relevant resource. Also, consult Bluebook Rule 21: International Materials for further guidance on sources and their citations.
For in-depth research, start by reviewing the information below then proceed to Secondary Sources.
1. The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
Public international law research involves locating evidence of some or all of the five key sources of public international law (from ICJ Statute, Article 38(1)) that apply to your legal issue, then applying these sources to your situation.
As with all legal research, the best first place to begin your research is generally by finding and reading reliable secondary sources that will familiarize you with the treaties, judicial decisions, custom, and/or general principles you'll seek to locate, read, contextualize, and understand in the second part of your research.
To find additional guides, search the web for:
Guides are often found on library websites, published as journal articles, or on blogs.