Macbook Pro late 2011 + Windows 10+ High Sierra
I was able to successfully install Windows 10 on my Late 2008 MacBook running macOS Sierra. I created a bootable Windows 10 USB key on another PC and performed a normal install on my MacBook.

As per official guidelines, Late 2008 MacBook does not support installing versions of Windows later than Windows 7 using Boot Camp. But I was able to find a workaround, thereby doing a normal PC installation.

The installer must be created on a pen-drive and not on a external hard-disk as it can create troubles.

I first created a Windows partition on macOS by resizing, plugged in the bootable pen-drive, restarted my MacBook, pressed Alt + r key till the boot device selection screen appeared. Then I performed a normal Windows installation.

Windows can overwrite the boot loader which can be repaired with the recovery partition.