For the fingerprint scanner to operate, hands must be clean and free of moisture, dirt, oil, lotion, dyes, etc.
Up to four fingerprints can be registered on the device.
From a Home screen, navigate: Settings > Lock screen & security.
Tap Fingerprints > Next.
If presented, enter the current PIN, password, pattern, etc.
Place your finger on the Power key to scan the fingerprint then follow the on-screen prompts.
Repeat this step as necessary until the process is complete.
For greater accuracy, finger placement should vary slightly each time.