Bootcamp - Windows 7 Installation Help
You have to install Windows 7 to BIOS boot. EFI booting is not possible for two reasons. First the EFI firmware in your Mac is not new enough. Second, the drivers are for a BIOS booting Windows. The Apple way is to use a the Boot Camp Assistant and a Windows 7 DVD. You can choose either a 32 bit for 64 bit Windows. Windows 7 should include SP1. You can download the current Windows 7 from Microsoft. If necessary, the Window Support Software (Boot Camp Support Software) can be downloaded from the Apple website Install Windows 7 on your Mac using Boot Camp.

Your model Mac can not BIOS boot from a USB port. Therefore, modifying the Boot Camp Assistant application to create a Windows 7 USB installer is a pointless waste of time.

If the use of a DVD is not an option, then the following methods can be used or adapted.

The VM method. Here, a virtual machine is only used to install Windows 7.
The GUI method. The answer is for Windows 10. You will have to adapt for Windows 7.
The CLI method. The answer is for a Windows 7 and 10 dual boot. You will have to adapt to omit Windows 10.