1. Ask inventor if they have any patent references; typically professors are more likely to cite journal articles rather than doing a patent search.
2. Search for key words; a typical goal might be to narrow the number of references to approximately 20.
3. Look to see if there is a pattern to fall into a given "class," or a given "subclass" among the references.
4. Search through more patents in that class/subclass with a more refined keyword search.
5. Ideally you will know the search has been thorough if you find a reference or two that is/are very similar to the disclosed invention that requires careful study to discern the differences.