To start a Mac in Safe Boot mode, follow these steps:

1. Shut down your Mac: Click on the Apple menu in the top-left corner of the screen and select “Shut Down” from the drop-down menu. Wait for your Mac to completely power off.
2. Start your Mac: Press the power button to turn on your Mac.
3. Press and hold the Shift key: Immediately after pressing the power button, press and hold the Shift key on your keyboard. Keep holding the Shift key until you see the Apple logo or a progress bar appear on the screen.
4. Release the Shift key: Once the Apple logo or progress bar appears, you can release the Shift key. Your Mac will now start up in Safe Boot mode.
5. Wait for Safe Boot to complete: It may take a bit longer than usual for your Mac to start up in Safe Boot mode. You will see “Safe Boot” displayed on the login window or on the desktop.
6. Use your Mac in Safe Boot mode: While in Safe Boot mode, your Mac will disable certain software and features, perform disk checks, and load only necessary kernel extensions. You can use your Mac as usual, but some functions and third-party software may not work correctly.
7. Restart your Mac to exit Safe Boot mode: Once you have completed the tasks you needed to perform in Safe Boot mode, you can restart your Mac to exit this mode. Click on the Apple menu, choose “Restart,” and let your Mac boot up normally.

By following these steps, you can start your Mac in Safe Boot mode, which can help troubleshoot issues related to incompatible software, startup items, or disk-related problems.