"Your iPhone’s Touch ID settings are located in the Settings app, under General > Passcode & Fingerprint. If you’ve already set up a passcode, you must enter it in order to gain access to this screen—you can’t use an already-registered fingerprint to open this section."
Touch ID is only one part of your iPhone’s security system. In setting it up, you must also set up a passcode (as a fallback identification system in case fingerprint recognition fails). If you choose not to use the fingerprint scanner, you can always swipe to the right on the lock screen at any time to access the passcode entry screen.
Rogers goes on to explain the process, which requires an unsmudged, complete print of the correct finger and a way to “lift” the print using cyanoacrylate (super glue) fumes, fingerprint powder, and fingerprint tape. The lifted fingerprint must then be photographed, edited, and printed onto transparency film, where it is converted to a usable fingerprint via a PCB board or a laser printer.