Its fairly simple...Most of the time OS is distributed in an .iso sometimes a .vhd if lucky like that download the .vhd. If not then you have to make one from the .iso
In Windows 7...follow this
Image to Virtual Hard Disk (WIM2VHD) Converter | IT Core Blog
In Windows 8...follow this
PowerShell: Create a Windows 7/8/2008/2012 VHD using ISO file
Once you create the .vhd (Name it something like Win10.vhd) Place it in any folder (I use C:\VM) and use the steps in the Windows 8 guide to add the proper boot entries to your BCD registry file..this will give you a dual boot situation after you add the proper entries.
Takes a bit of effort but the result is fantastic... Make a copy of the VHD so you always have a sysprepped copy of the .vhd ready to deploy if needed on any system.
Once done testing just delete the VHD. Your system will be back to normal.
In the meantime you can go back and forth between original os and any VHDs you have set up there really is no limit to how many other than space constraints.
Plus column for VHD....
No partitioning
Faster than VM...99.8% same as bare metal install
Lets you be booted fully into Windows 10...rest of drive available as D:
Clean removal
Expandable file takes up less space when not used.
Once made any copies are reusable on other computers. (Its what Hyper-V used for its VMS or XPMode)
Great for versioning...different set ups
Minus...
Takes converting .iso...some work some time to create
Not as easy as setting up a VM off an iso install.