Windows 7 install on MacBook Air without Bootcamp
I assume your MacBook Air is 2015 or newer model. If so, then you can not install Windows 7 on this model. The reason is that Windows 7 requires a BIOS boot method and Apple removed the ability to BIOS boot operating systems starting with the 2015 model year Macs.

The instructions for installing Windows 8.1 are given at the Apple web site: Use Windows 8.1 on your Mac with Boot Camp.

The instructions for installing Windows 10 are given at the Apple web site: Use Windows 10 on your Mac with Boot Camp.