Wiki

https://www.techrepublic.com/article/pro-tip-edit-boot-camp-to-install-windows-from-a-usb-flash-drive/

Download windows 7 from here: https://msdn.itellyou.cn. Use emule

Trouble Shoot

Bootcamp says “Boot Camp 安装失败: 拷贝 Windows 文件时出错”. This is because Windows ISO larger than 4GB cannot be loaded to USB drive. Download an earlier version smaller than 4GB