Here are instructions on how to wipe an old Mac – any Mac that is before 2018 with a Fusion Drive or a solid state drive (SSD).

Part 1: Encrypt the drive

1. Turn on the computer, and log in as usual.
2. Go to Apple Menu > System Preferences > Energy Saver. Check the box that says “Prevent Mac From Sleeping When Display Is Dimmed” or something to that effect. Also set the Display Sleep time to “Never.”
3. Click on the “Show All” button at the top of the System Preferences window (looks like a 4×3 grid of dots). Click on Security & Privacy > FileVault.
4. Turn on FileVault. If you are prompted to choose between iCloud or a recovery key, choose the recovery key. You do not need to save it, since we’re just going to erase the drive anyway.
5. You’ll see a progress bar indicating encryption status. 24-48 hours is normal, though it can take less. You’ll know it’s done when It no longer shows you the progress bar and it simply says that FileVault encryption is on.
6. Whenever you see that it’s all done, shut down the Mac from the Apple menu.

Part 2: Erase the drive

1. Before you turn your Mac on, get ready to hold down command-R on the keyboard, which you’re going to do as soon as you release the power button.
2. Press and release the power button, then immediately hold down command-R.
3. When you see the bar progressing underneath the Apple logo, you can let go.
4. If you are taken to the usual login screen, or your desktop, it didn’t work. Click Shut Down, and start over.
5. If it did work, you will be taken to the Recovery Mode. You may be asked to choose a user on the system, in which case, choose yourself, click Continue, and enter your password.
6. You should now be booted to the Recovery Disk, which you will see as a menu offering things like Time Machine, Install macOS, DiskUtility. Open DiskUtility.
7. Click on “Macintosh HD” on the left.
8. Click the Erase button. If the name appears as “Untitled,” change it to “Macintosh HD.” Set “Format” to “APFS” (the plain version, with no extra frills), and, if shown, leave “Scheme” as “GUID Partition Map.” Click Erase.
9. After it’s done, which will take between 30 seconds and 5 minutes, quit DiskUtility.

Part 3: Reinstall a fresh macOS

1. Back at the main Recovery disk screen with the options, click on the WiFi icon in the far upper right of your screen, and make sure it’s connected to your network.
2. From the main screen, choose “Reinstall macOS,” and follow the prompts.
3. Once it looks like it has begun, and shows a time estimate, it’s really only the beginning. The Mac will restart, and you’ll see the Apple logo for a long time, and then it may go through several more restarts like this. This is normal. When it’s all done, it will say Hello or Welcome or something else as though you’re using it for the first time.
4. After choosing country and keyboard, press Command-Q. A box will pop up giving you the option to Shut Down. Click Shut Down.

You’re done! Unplug the Mac.