The NY Times tech section shares an interesting article on how to install on an Apple iMac using a tool called “Boot Camp”

https://www.nytimes.com/2017/04/28/technology/personaltech/running-macos-and-windows

Boot Camp is Apple’s tool for the Mac.  It allows you to install and use the Windows 10 operating system on a separate hard drive partition — if your hardware meets the system requirements. According to Apple’s guidelines, the earliest iMac model that supports running Windows 10 is theLate 2012edition with either a 21.5- or 27-inch screen.

If your iMac passes muster for Boot Camp, make sure you have 55 gigabytes of free space on your start-up drive and an empty USB flash drive with a capacity of at least 16 gigabytes. You also need a copy of the 64-bit version of Windows 10, which you can buy as a download or on a USB drive from Microsoft.

Before you start installing Windows 10, make sure your Mac’s software is fully up-to-date and back up your computer just in case something goes awry. If you have not created a partition on the Mac before nor installed Windows on it, go to the Mac’s Applications folder and to the Utilities folder and open the Boot Camp Assistant program to get started.