You can download the Windows 7 ISO from the Microsoft Website and create bootable media, whether an optical disc or USB drive.
You would have to buy a license key for Windows 7 if the machine was from the factory with Windows 10.
The rest of the process is no different, no matter what "direction" you're going:
1. Take a full system image backup of your current system with your preferred system image backup tool
2. Take a separate backup of just your user data.
3. Get into UEFI or BIOS to reset boot order to look at your bootable media drive first.
4. Boot the machine and go through the installation using the bootable media and license key.
All of the above being said, I do not recommend downgrading from Windows 10 to Windows 7 at this point in time. Official support is running out, quickly, on Windows 7 and January 2020 will be here before we know it.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall