How do you set up dual monitor computers?

Please take me through the process of setting up a dual monitor computer system. Could you give me a step-by-step guide on how to assemble a computer with dual screen? What things do I need to pay particular attention to in order to ensure that everything turns out just right? How do you run a test drive to ascertain whether the system is working or not? What sign should I look out for as indicative of a successful set up?
