You will get an Install Succeeded message and a restart button.

It will give you an estimate while installing and can be anywhere from 10-30 minutes. Look through the other options and check what you think you might need.

In the customize screen the most important thing to check is one of the boot loaders.

Continue the installation but make sure to click Customize on the bottom left when that screen comes up.

Make sure the format is OS X Extended (Journaled) and set a name like OSX. Choose the hard disk click the Erase tab. If all has been setup correctly you should come up to installation screen. You might need to press F8 to enter the boot loader or boot from the CD. Add the MAC OS X 10.8X Mountain Lion ISO as a CD/DVD Device. Add the VDI you had created during the new VM wizard or you could also create a new one. Delete the SATA controller and VDI in the list. Increase the Video Memory as high as it will go which should be 128MB. System is our first place we need to make changes.

You will see all of the different options on the left side.

Select your new VM that you just created and click Settings.Hard drive size is up to you but do not go below the default 20GB.Choose Dynamic or Fixed then click Next.

VDI is the default which is fine, click Next.Make sure Create a virtual hard drive now is selected and click Create.There has been issues sometimes if more than 4096 is selected. Memory can be left at 2048 but if you have more to spare you can increase it.In the name type MAC OS X Mountain Lion or whatever you want and make sure the Type is MAC OS X and the Version is Mac OS X 10.8 Mountain Lion (64 bit).

Virtualbox for mac os x mountain lion