MAC Erase and Install
To Erase Hard Drive:
Put original disk or usb in computer
Press and hold the “C” key while the computer starts up.
Under utilities menu, choose disk utility
Select a volume to erase (usually called Macintosh HD)
Click the erase tab
After partition is erased, choose quit disk utility
Installing the new OS
When the installed (from either the usb or CD) appears, click continue
Agree to the license
Select the disk you just erased. (It should be highlighed with a green arrow)
Click install and continue as needed.
An install succeeded screen will appear. Click Restart.