You can easily download OS X Mountain Lion from the App Store any time after purchasing it, but if you want your own physical copy of the installation, here’s Simple 6 Step to Guide how to Install Mountain Lion and burn DVD or USB drive
Follow This Simple 6 Steps:
Step 1: Download Lion from the Mac App Store,but don’t install at this time.
Step 2: Find the installer in your Applications folder and select “Show Package Contents.” Head to Contents > SharedSupport and See the file called “InstallESD.dmg”
Step 3: Open up Disk Utility from your Applications > Utilities folder and drag the DMG file into the sidebar.
Step 4: If you are burning to DVD insert your disk, select the DMG file in the sidebar, and press Burn.
Step 5: If you’re burning to USB, you will need to format the drive properly:
- Insert the disk and select it in the sidebar in Disk Utility. Select the Partition tab, select “1 Partition” from the dropdown menu and choose “Mac OS Extended (Journaled)” for the format type.
- Click the Options button and choose “GUID Partition Table”—this will make the drive bootable and formatted correctly for the Mac. Hit Apply to format—which will completely erase—the drive.
- Now click the Restore tab and choose the DMG file as your source and the USB drive as the destination. Press the Apply button and Done!
Step 6: Finally, to boot from your install disk/drive and install Mountain Lion, restart your Mac and hold the Option key down when you hear the startup chime.