Open the Find My app on your iPhone
Sign in with your Apple ID if prompted
Tap Devices
Select your iPhone from the list
View its location on the map
Tap Play Sound to make your iPhone ring
Tap Directions to get navigation to its location
Tap Mark As Lost to lock the iPhone and display a message
Enter a phone number and message if needed
Tap Activate to confirm Lost Mode
Tap Erase This Device if you need to wipe the iPhone remotely
Use iCloud.com/find on another device if you do not have access to the Find My app
Make sure Find My iPhone is enabled in Settings > [your name] > Find My > Find My iPhone