To turn on Find My for your iPhone:

1. Open the Settings app.
2. Tap your name, then tap Find My.
3. If you want friends and family to know where you are, turn on Share My Location.
4. Tap Find My [device], then turn on Find My [device].
5. To see your device even when it’s offline, turn on Find My network.*
6. To have the location of your device sent to Apple when the battery is low, turn on Send Last Location.

If you want to be able to find your lost device on a map, make sure that Location Services is turned on. To do this, go to Settings > Privacy > Location Services, and turn on Location Services.

From Apple Support’s official website.

This is also the process for adding the Find My function on iPad, or iPod touch.