Create a distinction between the good decent people who are our the basis for our good lives, and the vial pests who threaten our way of lives. We want them gone.
Step 2: Obey Orders
Create a society where authority is obeyed. This obeying of authority for the majority of the people is second nature. This has been done in many societies since childhood.
Step 3: Do harm to "Them"
Have people do harm to this other group even when against it may be against their better judgement using a combination of Step 1 and Step 2 to create a situation where this begins. This has been proven possible to happen by an experiment of Stanley Milgram's when he got volunteers to believe they were electrocuting someone else. No electrocution happened and was simulated but was to see if the volunteer would do it. And 2/3 of 900 American volunteers (who knew nothing of the nature of the experiment) went up to 450 volts which was the maximum.
Step 4: "Stand up" or "Stand by"
This is the most crucial part, and... can be summed up by the words with the words "All that is necessary for the triumph of evil is that good men do nothing." - Edmund Burke & Leo Tolstoy
This is often pointed towards the rise of Hitler and only if people were to stand up in numbers, it wouldn't have happened. Though seemingly small, it is the most important part.
Step 5: Exterminate
With the combination of the previous 4 steps a tyrannical regime has been created and all that is needed is to aim it and hope for the "benefits" to be reaped. I hope this is self explanatory though the examples. Given in the