Official steps to turn off Find My iPhone Activation Lock are documented by Apple at https://support.apple.com/en-us/HT201441
In a nutshell, the various ways to remove iCloud Activation Lock include:
- Apple will remove the lock if they can verify the original purchaser is requesting the action.
- Self-service removal if you can authenticate as the iCloud account that placed the lock on that device.
- Business using DEP can often use their MDM to remove the lock as a self-service action without needing Apple to be involved.
For consumers, you will likely need to make an appointment at an Apple Store and provide the Genius on-hand proof of purchase (to show you're the authorized owner) along with proof of identity and they will reset Activation Lock, giving you access to the device.
For businesses, it depends if you are enrolled in DEP - Apple's Device Enrollment Program or just want to provide copies of the original purchase listing serial numbers to verify ownership.
If you bought the device second hand, stole it, found it or came to possess the device by some means other than buying it from Apple directly, then you will probably need the original owner to show proof of purchase, etc. They will then transfer the device to your account or simply disable Activation Lock.
The only self-service way to remove the lock if you can't type your password into the device itself is to authenticate at https://icloud.com and remove the activation lock when the device is not connected to the internet.