Press the side button once to lock the iPhone
On iPhone with Face ID, press the side button
On iPhone with Touch ID, press the top button or side button
Wait for Auto-Lock to turn off the screen automatically
Go to Settings
Tap Display & Brightness
Tap Auto-Lock
Choose a time for automatic locking
Set a passcode in Settings
Tap Face ID & Passcode or Touch ID & Passcode
Turn on Find My iPhone for remote locking
Use Find My on another device or iCloud.com to lock it remotely
Put the iPhone in Sleep mode by pressing the lock button