I have windows 10 on an external disk and I use the 'Option' key at boot to select it.
I prefer this method as I can keep the Mac and Windows environments completely seperate.
Other methods are BootCamp or Parallels.
I usually recommend Parallels or Fusion, as they don't require a restart, and are easier to install/setup. You will need to purchase Paralells/Fusion, as well as a copy of the version of Windows you want to install.
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