Here simple alternate to a Failure Mode and Effects Analysis (FMEA) by Gary Klein:
Step 1: Preparation. Team members take out sheets of paper and get relaxed in their chairs. They should already be familiar with the plan, or else have the plan described to them so they can understand what is supposed to be happening.
Step 2: Imagine a fiasco. When I conduct the PreMortem, I say I am looking into a crystal ball and, oh no, I am seeing that the project has failed. It isn’t a simple failure either. It is a total, embarrassing, devastating failure. The people on the team are no longer talking to each other. Our company is not talking to the sponsors. Things have gone as wrong as they could. However, we could only afford an inexpensive model of the crystal ball so we cannot make out the reason for the failure. Then I ask, “What could have caused this?”
Step 3: Generate reasons for failure. The people on the team spend the next three minuted writing down all the reasons why they believe the failure occurred. Here is where intuitions of the team members come into play. Each person has a different set of experiences, a different set of scars, and a different mental model to bring to this task. You want to see what the collective knowledge in the room can produce.