We don't recommend going back to Windows 7. In stead our advice is to download and install Classic Shell (free), set it to show the Windows 7 Start Menu and you're ready to go in something that looks very much like Windows 7 and has the advantages of Windows 10: faster, safer and longer supported.
However, if you're sure you want the real Windows 7, the steps are:
- download the installation media from https://www.microsoft.com/en-us/software-download/windows7
- buy a license key
- boot from the installation disc or stick you made
- answer the questions
But what are your reasons?
I know it is a low question, but is headache for me, is there a simplest way to install it? not many step, my system is windows 10 now.