My FAQ shows a detailed walk-through here at Windows IT Pro's website.
Just replace Windows 7 with Windows 8 media.
If this is an existing machine, and you want to dual boot rather than wipe the disk, then don't perform the first three blocks of commands. The process will be as follows:
1. Boot from the Windows 8 media (USB, CD, etc.).
2. At the Install screen, press Shift+F10 to open a command prompt.
3. Start diskpart and create the VHD file you want to install to:
select vdisk file=c:\win8.vhd
create partition primary
format fs=ntfs label="Win8" quick
4. Exit diskpart, and when you install Windows 8 you’ll be able to select your VHD volume (ignore the warning that you won't be able to install).
To see more FAQs, please go to John Savill's FAQs page on Windows IT Pro.