Primary sources: Treaties, Customs (recognized and invariably practiced) & General principles of law recognised by most nations.

Secondary sources: If, concerning a particular matter, no substance is found in treaties, customs and general principles, then secondary sources are consulted i.e. judicial decisions and teachings of publicists.

Furthermore, there is no hierarchy in primary sources as such. Usually it goes like this: if a treaty exists (between disputing nations) then that treaty trumps everything. And if a recognized custom has been established in a certain area then that custom will determine legality or otherwise of any matter in that particular area of Int Law.

I hope this helps.