Apple has just recently released a final public build of OS X 10.8 Mountain Lion. Anyone previous OS X users can purchase the upgrade package via App Store while Mac users who bought a Mac within the past month can grab it free of cost. You can simply do the upgrade to make things simpler but in case if you want to do a fresh OS X install, simply read these steps to make clean OS X 10.8 installation safely.
How To Clean Install
1- Create a bootable OS X 10.8 Installation DVD/USB (read how-to)
2- Connect your newly created installation DVD/USB drive into your Mac
3- Reboot your Mac and hold down the Option key
4- From the boot menu, choose “Mac OS X Installer” startup volume
5- Select “Disk Utility”
6- Then choose the hard drive you wish to format
p.s: The process described below will format the selected Mac disk and erase everything on it. A clean install removes everything: all apps, all files, everything is formatted. That’s why you need to backup our data first.
7- Click on the “Erase” tab then pull down the “Format” menu
8- Now select “Mac OS Extended (Journaled)” as the type
9- Then simply name that drive to something like “Mountain Lion”
10- Click the “Erase” button and wait till the format completed
11- Once the format process completed, close/exit Disk Utility
12- Select the “Install Mac OS X” option from the menu
13- Choose your freshly formatted hard drive
14- Then do install Mountain Lion
15- Once done, your Mac will reboot.
That’s all. Once your Mac reboots to Mountain Lion, you can then restore your backup. If your previously installed apps are stored in your Purchase History through the App Store they could be re-downloaded.
If we summarize all the steps above into simpler steps:
1- Backup data to TimeMachine / External harddrive
2- Clean all the trash file
3- Do the clean install of Mountain Lion
4- Restore from time machine