How to clean or fresh install iOS?

I bought iphone 4S 16gb. I unlocked my 4s via factory unlock in my desktop. So it's not jailbreak. My current OS is ios 5. I want to update it to ios 6. How do i clean install my iOs 6? Please give me step by step. I mean this updating has similar to windows? It possible to clean install your ios 5 and make it to ios 6. I wan't it to clean install because i wan't to save my storage. I mean no double Ios on my 4s is running.

Go to itunes, then click iphone, then check for update, and it should do its thing.