Install VirtualBox 4.3.12, or whatever the latest version is. Make a new VM image. You can use either 'Mac OS X (64 bit)' or the 10.8-specific one -- they both seem to work.
The world’s most popular cross-platform virtualization software enables you to, PC, Linux, or Oracle Solaris machine. Oracle VM VirtualBox for Mac is a general-purpose full virtualizer for x86 and AMD64/Intel64 hardware, targeted at server, desktop and embedded use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software. Some of the new features of Oracle VM VirtualBox are: Paravirtualization support for modern Windows and Linux guests Oracle VM VirtualBox is able to expose a paravirtualization interface to facilitate accurate and efficient execution of software. Once the virtual machine platform is defined, Oracle VM VirtualBox improves guest OS performances by leveraging built-in virtualization support (KVM on Linux guests and Hyper-V on Windows guests).
XHCI controller to support USB 3.0 devices Guest operating systems are now able to directly recognize USB 3.0 devices and operate at full 3.0 speeds. Guest can now be configured to use USB 1.1, 2.0 and 3.0 devices. Improved Drag & Drop support Starting from Oracle VM VirtualBox 5.0 improved the support of a bi-directional drag&drop between guest and host operating system; drag and drop support is available with latest guest-additions installed. Disk image encryption Oracle VM VirtualBox 5.0 allows for encrypted virtual disk images by leveraging AES algorithm in XTS mode (128-bit or 256-bit); since the DEK is stored as part of the virtual machine configuration file, encryption introduces a further security feature that will ask for a password while starting the virtual machine. Headless and Detachable start options Oracle VM VirtualBox now supports starting virtual machines in the background with a separate front-end process that can be closed while the virtual machine continues to work. Also Available.
Your virtual machine will now be created. But don't stop now-you still need to change a few settings before your machine will actually work. Your new virtual machine will show up on the left column of the Virtualbox start page. Select your Mac OS X virtual machine (single-click) from the main page of Virtualbox, and open up the virtual machine settings. Once the settings open up, go to 'System' and uncheck the 'Enable EFI' box. This is by far the most important single setting that you will need to change. You can change '1920x1080x32' to whatever resolution best fits your monitor.
For instance, if you want to use the 1600x900 resolution, type in '1600x900x32'. Once you've saved it, turn off the virtual machine. Next, open the Command Prompt in Windows (make sure you are logged into an Administrator account on Windows). You can do this by opening the Start Menu, and typing 'command prompt' into the Start Menu search bar. Then, type the following command into the Command Prompt. Cd 'C: Program Files Oracle Virtualbox' This command will change the focus of the Command Prompt to the program folder for Virtualbox (if you installed Virtualbox somewhere different, then change the command to wherever you installed it). Next, type in this command: vboxmanage setextradata 'Name of virtual machine' 'CustomVideoMode1' '1920x1080x32' This command activates 'vboxmanage', a command-line program included with Virtualbox that allows you to edit the properties of your virtual machine, including its built-in resolutions.
Replace 'Name of virtual machine' with the name of your virtual machine- you can find the name of your virtual machine in the left pane of the Virtualbox main window (in the screenshot below, my virtual machine is named 'Mountain Mac 2'). Replace '1920x1080x32' with whatever resolution you're using.
Recap I'll just repeat what I said in my Snow Leopard guide. Installing Mac OS X on a virtual machine is excellent practice for the real thing: installing Mac OS X on your actual computer. Don't get too comfortable, though. Compared to most computers, Virtualbox virtual machines are very 'vanilla', meaning that they're very compatible with Mac OS X in the first place. After all, ethernet works from the start.
You can't count on being that lucky with a real PC. And even if you don't plan on doing this for real, with a Hackintosh, it's still a really cool thing to try out over the weekend.