Um, the normal way. Boot from pre-prepared bootable USB installation image. You can purchase Windows releases ready to go (on a USB flash disk) or you can purchase only the License Key and download the image from Microsoft and make the USB flash disk yourself (I use Rufus for this myself, but Microsoft has a tool you can download for doing this).
Once you have installed Windows, you need to download and manually install the latest drivers for your NUC. Got to this page: Downloads for Intel® NUC Kit NUC5CPYH and download all of the latest driver packages for your version (presumably 64-bit Windows 8.1). Because of issues, I do not recommend the use of Intel's Driver Update Utility.
Hope this helps,
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