1. Using a simple combo of keys is the easiest way to instantly lock screen on Mac. 

2. Ctrl + ⌘ + Q. Just make sure to press the keys carefully when using this shortcut to lock Mac, so that you don’t hit just ⌘ + Q by accident. Without the Ctrl key, it will result in shutting down the app you’re working with, which makes it more likely to lose any unsaved work.
3. Ctrl + Shift + Power. A simple Mac lock screen shortcut to send your display to sleep.
4. Ctrl + ⌘ + Power. Another good option to consider, which sends your entire Mac to sleep as well but also saves more energy by turning to low-power mode and stopping background tasks.

Complete instructions from Mac here. 

There are multiple ways to lock your PC when you leave its vicinity.

1. Windows-L

Hit the Windows key and the L key on your keyboard. Keyboard shortcut for the lock.

2. Ctrl-Alt-Del

Press Ctrl-Alt-Delete. On the menu that pops up, click Lock. 

3. Start button

Tap or click the Start button in the bottom-left corner. Click your user icon and then select Lock.

4. Auto lock via screen saver

Go to Control Panel > Appearance & Personalization > Change screen saver and then check the box for On resume, display logon screen.

1. Go to Settings > Touch ID & Passcode. On devices without Touch ID, go to Settings > Passcode. If you have an iPhone X, go to Settings > Face ID & Passcode.
2. Tap Turn Passcode On.
3. Enter a six-digit passcode. Or tap Passcode Options to switch to a four-digit numeric code, a custom numeric code, or a custom alphanumeric code.
4. Enter your passcode again to confirm it and activate it.

Complete instructions from Apple here. 

1. Go to the Settings menu on your device.
2. Scroll down until you find “Security” or “Lock Screen and Security” and tap it. This is typically located under the “Personal” section on Android 4.2 or higher.
3. Under the “Screen Security” section, tap the Screen Lock Type option. By default, this option is set to “Swipe,” which means no password or pattern is needed.

From here, select which lock type you want to use:

1. Pattern: swipe to draw an unlock pattern you want to use. If you messed up, hit retry. Otherwise, hit continue. It will then ask you to draw that pattern again to confirm.
2. PIN: insert a 4-digit PIN that you’re comfortable with. It’s a good idea not to use repeating numbers (don’t use more than two if you absolutely must). Hit continue. Re-enter the PIN to confirm.
3. Password: type the password you’re comfortable with. The password must be at least 4 characters, must be no more than 17 characters, and must contain at least 1 letter. Use an alphanumerical password with symbols that’s at least 8 characters long for ultimate security, but anything will do. Hit continue. Re-enter the password to confirm.
4. Fingerprints: If available on your phone, a PIN or Password must be set as a backup before you can enable Fingerprint Security.

Complete instructions from Google here.