how to write a good paper review?

I just started my master's in computer science, and I started working on a research project with one of my professors last term as a part of a class project for his class. In relation to that project he has asked me to help him review a paper that is being reviewed to determine whether it will be accepted or rejected to a conference. I have agreed to review the paper. However, I have never written this type of paper review before. What are the steps you go through in reviewing a paper to determine if it should be accepted or rejected ?