First you need to change a few BIOS options. Check your computer manufacturer's owners manual to find out how to access the BIOS for your particular computer. Enter BIOS, then:

1. DISABLE secure boot option (only supported in Windows 8 and 10)
2. ENABLE legacy boot mode.
3. Change boot device sequence priority. Set 1st boot device to the DVD/USB which contains your Windows 7 installation media.