Restoring a Nandroid Backup
1. Boot into TWRP Recovery
2. Go to Restore Menu and select the backup you want to restore.
3. Select the partitions you want to restore.
4. Swipe to Restore. The process will take 6 to 10 minutes.
5. Reboot system once done
Note: If you happened to forget to remove lockscreen security (PIN/Pattern/Password) before creating your backup, and you cannot get into your OS after restoring, you can do the following to fix the issue.
Boot into TWRP
Use the TWRP File Manager and Navigate to /data/sytem/ and delete the following files:
You may or may not have all the above files so delete the ones you have.
Reboot the phone and (if you've set a PIN) enter it to decrypt the storage one more time. After that you can simply unlock your phone with a swipe.
Upon Boot > Go into Settings > Security > set your preferred unlock method
Android will ask you if you want to set a boot-time code too so select as per convenience.