For the fingerprint scanner to operate, hands must be clean and free of moisture, dirt, oil, lotion, dyes, etc.
Up to 4 fingerprints can be registered on the device.
From a Home screen, navigate: Settings > Lock screen & security.
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.