Depending on what version of Windows you want, there are different ways of getting it installed. I would suggest getting a Windows 10 license and installing it, as it's a more modern OS (not to mention it's the newest) and far more secure than Windows 7. Microsoft even has a free tool that lets you create a bootable media to install W10 and a compelling artice about how that's done: https://www.microsoft.com/en-us/software-download/windows10
This article here from WikiHow is about installing Windows 7: https://www.wikihow.com/Install-Windows-7-(Beginners)