How to jailbreak iphone?

Got this iPhone from US through a friend a month ago. Earlier I upgraded my phone ie iPhone 3GS to the version 4.3.5. I want to know how to jailbreak this iPhone. Is there a way I can do it myself. Or it would be great if you guys can suggest a way by which I can get my iPhone to jailbreak for free of cost. Please tell me in simple steps how I can do it myself. There must be a way that still works for jailbreaking these phones.