You download and install VirtualBox. You define a new virtual machine (File -> New) for the operating system you want to run as a virtual machine. You define it's memory, ethernet, HDD size, etc. You attach the operating system ISO file to the virtual optical drive of the new virtual machine you created and then you start it. It installs the operating system as normal and now you have an operating system in a window in your physical operating system.
That's it. Hardly wizardry.