Go to Settings > Touch ID & Passcode
If promoted, enter your passcode
Click Add a Fingerprint
Follow the steps to add your finger print.
Ok, you have added your fingerprint. Normally, you would stop here, but we are going to repeat this process again using the SAME FINGER.
That's it! The more you repeat the process, the more reliable your finger print becomes (you can add up to 5 fingerprints). Enjoy unlocking bliss!