Open Settings > General Tab > Fingerprints & Security > Fingerprints.
Note: To register fingerprint, you need to first set security lock (knock code, etc.) to be used with fingerprint.
Enter your lock screen protection to begin registering your first fingerprint on your LG G6.
Lightly touch the power button multiple times to register your fingerprint.
When complete, tap OK to exit or tap ADD MORE to register additional fingerprints.
Note: Up to 5 fingerprints can be registered.