If you just started to do this now, the best guide I can find is HERE. However, there are two places that might give you troubles. So when you are following the guide, keep two things in mind.

1). in step 3. After reboot, create new Guest OS on VirtualBox and choose MAC OS X server
10.7.2 does not show MAC OS X server in VirtualBox 4.2.4. However, both MAC OS X and MAC OS X (64 bit) will work. I have both installed and running well. (See pic)
2). After 16. choose our installed partition for booting using arrow key and press Enter
Your screen might seen to freeze with a process ring keeps spinning forever. May be you are thinking “oops, this does not work”. Don’t panic, you have the ubiquitous problem solving tool for Windows–reboot. No, do not reboot your Windows. Just reset the OS X you installed in VirtualBox menu.

You are done.

VirtualBox wins VM Player this time. I was not able to install it on VM Player.