There are two methods for performing a clean install of OS X Mountain Lion. If you're planning to install Mountain Lion on your Mac's startup drive, read on.
If the target of your Mountain Lion installation is anything but your startup drive, then you need the How to Perform a Clean Install of OS X Mountain Lion on a Non-Startup Drive guide.
Start Your Mac From the Bootable Mountain Lion Installer
If you're going to install Mountain Lion on your Mac's startup drive, you must first restart your Mac from a bootable copy of the installer. If you haven't yet created a bootable copy of the installer, you'll find instructions in the Create Bootable Copies of the OS X Mountain Lion Installer guide.
You must start your Mac from the bootable media because you must erase the startup drive before you perform the installation. You can do this using Disk Utility, which is included with the installer.
- Insert the bootable media, or connect it to your Mac, and then restart your Mac while holding down the option key. This will cause your Mac to display its built-in startup manager, which will allow you to select the device you want to boot from. Use the arrow keys to select the bootable Mountain Lion installer you created earlier, then press the enter key to start the boot process.
- The Mac OS X Utilities window will display, as if you had booted from the Recovery HD partition. Of course, there isn't a Recovery HD partition available yet, because we haven't installed the OS. That's why we made our own bootable media.
- Select Disk Utility from the list of options, and click Continue.
- When Disk Utility opens, select your Mac's startup volume from the list of devices. If you never changed its name, the startup volume will be listed as Macintosh HD. Be sure to select the volume name and not the device name, which will usually be the name of the physical drive, for instance, "500 GB WDC WD5."
- Click the Erase tab.
- Make sure that Mac OS X Extended (Journaled) is selected in the Format drop-down menu.
- You can give the startup drive a name, or use the default name.
- Click the Erase button.
- You'll be asked if you are sure you want to erase the drive. Click Erase.
- Select "Quit Disk Utility" from the Disk Utility menu.
- You will be returned to the Mac OS X Utilities window.
- Select Reinstall Mac OS X from the list, and click Continue.
- The Install OS X window will open. Click Continue.
- A sheet will drop down, informing you that your computer's eligibility will be verified before you can download and restore OS X. This happens because the bootable media we created doesn't contain all of the files necessary for the install. The installer will check for any missing or new files it needs, download the files from Apple's servers, and then start the installation process. Click Continue.
- Read through the license, and click the Agree button.
- You'll need to click the Agree button a second time, just to make sure you really agree with the license, and didn't accidentally click the Agree button the first time.
- The installer will display a list of drives that you can install Mountain Lion on. Select the target drive (the startup drive you erased in the steps above), and click the Install button.
- The installer will check the Mac App Store for updates and any other files it needs. Enter your Apple ID and click Sign In.
- The installer will copy the necessary files to the target disk, and then restart your Mac.