Job at a PSU is the ultimate dream of someone with little ambition, one who is satisfied with little work, no competition and foolproof job security. However given that the median age of the Indian population is in the early 20s so the competition for every post every seat is very stringent. Thus it helps if you can piggyback on a good college or university. PSUs recruit people through either a written test (open the employment newspaper and you would find myriad of jobs with their examination schedules) OR through direct recruitment from campus. Given that law school people lose steam after CLAT, to pass the entrance exam for PSU would be impossible for most (many complaint that National law school people do not want to become judges, I suspect partially the reason would be that they are not confident of passing the exam or those who sit for it never pass). However coming to the point at hand, in this post I would give you a step by step method of how to crack a PSU job at the law school:
Step 1: Have more than 60% marks (or the CGPA equivalent of first class), PSUs being PSUs would have to follow certain rules so the first is that the candidates must have 60% marks, certain PSUs like CIL, NHPC, NTPC reduces the cutoff by 5% for reserved seats but certain others like ONGC, GAIL, SAIL are strict on this criteria. Thus if you do not have 60% there is no point sitting for PSUs.
Step 2: Usually there is a small test for the candidates to take, the test is very basic having questions like what is difference between pledge and hypothecation; what is insurable interest etc. A very basic and cursory reading of the Contacts Act etc. would be more than sufficient. This test is just to get cut off non-interested people and in never taken into account when deciding the final list.