Step By Step
You will need a USB stick of OVER 4GB ( I used an 8 GB stick for my copy)
Download ML from your purchased tab but cancel the installation when prompted.
Download:
http://www.cultofmac.com/180518/sin...on-diskmaker-gets-os-x-mountain-lion-support/
Insert the USB stick and start the creator application.
When complete (it takes a while with USB 2) You can eject the USB stick and you now have a bootable image,
Start your MAc whilst holding down the "Option" key
If you wish to, you can erase the drive to start afresh, or perform an (EFI) over the top installation of ML
Over the top will retain all your Apps, a clean install will require you to download and re-install them from the App store. If you have third party apps, make a note of them prior to the Installation