There is no single manual on how to unlock cellphones for free. Not all cellphones can be unlocked, and those that can do not have a single method that works across all devices. The unlock process varies depending on the phone manufacturer and model.
Phones based on CDMA technology cannot be unlocked due to technical limitations. All GSM phones have the physical capability to be unlocked, but in certain cases wireless carriers can legally block this capability. As of 2015, the FCC allows carriers to refuse unlock requests for postpaid phones under contract or in financing, or for prepaid devices with less than one year of service.