How to find a lost iPhone that is switched off using Find my iPhone?

Unfortunately, it is impossible to track the phone that is turned off and offline.

Put the device in the lost mode, so that it becomes useless for a thief next time it connects to the Internet.

Just FYI, take a look at these instructions and try to follow them carefully.

https://support.apple.com/en-us/HT204315

https://support.apple.com/en-us/HT201472

https://support.apple.com/kb/PH2701?locale=en_US