Please answer the question within 30 min (citing all sources used to make up the answer)

NOTE: refer to relevant Articles, relevant cases, legislations, declarations, Vienna Conventions on the law of treaties 1996 (VCLT) and statutes, (ICJ/ ICCPR) or textbooks such as 'The internationalists by Hathaway and Shapiro' as well as 'Crawford 2019' within your answer. (PLEASE USE IN TEXT CITATION)

QUESTION:

Analyse the requirements of custom that make it a valid source of international law. Please note that references to relevant cases/examples will add value to your answer.