The fastest way to lock your screen on your non-touch bar Mac
If you need to keep your work secure, it's important to keep your computer locked whenever it's unattended. The obvious way to lock your computer is to log out, but logging in and out is excessive and time consuming. The easiest way protect your work is to lock your screen with a password whenever the screen saver starts. Then configure a shortcut to start the screen saver. This guide will walk you through this process step by step.1
Open System Preferences > Security & Privacy.
Make sure "Require password" is checked and the dropdown says "immediately".2
Ctrl + Shift + Eject is the keyboard shortcut to lock the screen. This will require a password whenever the screen is unlocked.