Keep the machine offline to prevent network-based exploits.
Use third-party security software that still supports Windows 7.
If you want, I can:
The most secure way to download a legal Windows 7 ISO is to use a PowerShell script that fetches the original files directly from Microsoft’s servers.
Click . Rufus will wipe the USB drive and create your bootable Windows 7 installation media. Crucial Safety Warning for Windows 7 Users