If you want a greater depth of information, see Step 6. This is meant as a reference guide and so I've tried to avoid extensive explanations. This isn't so much an Instructable as it is an Informable. My objective here is to give you all the information that you might need in order to hack an ATX PSU to meet your needs.
We like to improve on what already exists and fit the product to our needs. My observation is that most makers, hackers, and mad scientists don't follow instructions very well.
There are tons of Instructables on how to hack a discarded ATX (computer) power supply unit (PSU) into a lab bench power supply.