You will need:
- A Mac running OS X 10.7 - Lion
- To have bought OS X 10.7 - Lion in the Mac App Store (unconfirmed)
- I also strongly recommend backing up your hard drive (be it Time Machine, or whatever)
Firstly, and obviously, you'll want to backup and make sure you have another copy of all your vital files.
Step 1: Boot into Lion Recovery Mode
Restart your Mac, and hold Command + R (â + R) as it powers back on.
This should take you into Lion Recovery Mode (pictured).