Download, but don't run, the Sierra installer from the Mac App Store. This places the installer at/Applications/Install macOS Sierra.app/.
How to hide menu bar on mac full screen. As Stephen Hackett of 512 Pixels notes. This means there's no easy way for someone to install macOS Sierra at this point. If your Mac came with 10.12.4 or greater, you can reinstall that version from Apple's servers, but going back from High Sierra isn't doable unless you've got a copy of Sierra laying around. Extract the Image File with Winrar or 7zip. Go ahead and install WinRAR. (I have included the latest.
Now run the following commands to build a suitable VM image from the installer:
Now transfer the following files to Windows:
osw-vm-templates/packer/macOS_10.12.ovf
osw-vm-templates/packer/macOS_10.12-disk001.vmdk
Mac Sierra Install Unapproved Apps
In Virtualbox File->Import Applicance. Select the OVF file generated on the Mac. Accept all the defaultsto import the appliance and its disk image.
Mac Sierra Install Unapproved Application
Don't start the image yet!
Mac Sierra Install Unapproved App Download
In a console issue the following commands to modify the image so it thinks it's running on a real Mac: