Swipe down from the top of the screen to access Quick Settings > Tap Settings icon > Security & lock screen > Fingerprint > tap Unlock device
Turn on Screen off unlock.
Once you have turned it on, you can simply place your finger on the spot where the onscreen scanner is. A circle will light up and show the exact spot if your placement is slightly off. Then it will quickly verify your fingerprint and unlock your phone simultaneously.
If you have checked the Tap screen to show indicator, tap the screen when it is off and the indicator will light up to show you the exact spot to place your finger.
If you have checked the Pick up phone to show indicator, pick up the phone when the screen is off and the indicator will light up to show you the exact spot to place your finger.
Note: If you have turned on "Lift to check phone" (Settings > Advanced > Gestures > Lift to check phone), the screen will also light up when you pick up the phone.