These steps cover erasing and re-installing older (Intel, prior to about 2021) Macs. New (Apple Silicon) Macs have a quite different (and much simpler) process. Do this if you are selling an old machine, or if want to restore it after any major fault.
Before erasing, you should normally do these steps: Back up all important files (the following will wipe your entire hard drive). Sign out of iCloud (Apple ID), the App Store, and iMessage. Unpair any Bluetooth devices (wireless mouse, etc).
Boot into macOS Recovery Mode: Restart your iMac (Apple menu, then "Restart"). Immediately press and hold the Command and R keys. Release the keys when you see the Apple logo or a spinning globe.
Erase the disk using Disk Utility: Once in recovery mode, you will see a "macOS Utilities" window. Select "Disk Utility" and click "Continue". In the "Disk Utility sidebar", select your main hard drive (usually "Macintosh HD"). Click the erase button in the toolbar. In the erase dialogue box, enter a name and choose a format: Use APFS for newer systems or Mac OS Extended (Journaled) for older systems. Note 1: If there is an option for secure erase, use it (it will take a lot longer). Note 2: For solid state drives, refer to my other tips on secure erasing these. Click "Erase" and then "Done" when the process is complete. Quit Disk Utility from the menu bar to return to the main utilities window.
Reinstall macOS: From the macOS Utilities window, select "Reinstall macOS". Click "Continue" and follow the on-screen instructions. Select the erased hard drive as the installation location. The Mac will connect to the internet to download and install a fresh copy of the OS.
Finish: Once the installation is complete, the Mac may restart to the Setup Assistant screen. If you are selling or giving away the iMac, press Command-Q to shut it down. The new owner can then complete the setup.
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