Open the Find My app on your iPhone
Tap Devices
Select the other iPhone from the list
View its location on the map
Tap Play Sound to make it ring
Tap Directions to open Maps and navigate to it
Tap Notify When Found if the iPhone is offline
Use iCloud.com/find in a web browser if you do not have the app
Sign in with the Apple ID linked to the other iPhone
Check that Find My iPhone is enabled on the other device
Make sure the other iPhone is powered on and connected to the internet
Use Lost Mode if the iPhone is missing
Contact local authorities if the device is stolen