Since you installed Win8 yourself, I don't know what sort of hardware you have. A new Win8 PC would be UEFI-based, Secure Boot would be enabled and the hard disk would most likely be GPT formatted. In any case, all you need to do is ensure Secure Boot is turned off if available, options like Legacy BIOS or Compatibility Support Module (CSM) are not enabled under UEFI settings (ignore if your PC lacks UEFI and has only BIOS), then boot from the Win7 DVD. The 64-bit WIn7 DVD supports both BIOS and UEFI, so as long as you boot using the appropriate mode it will install as usual on either system type. You can also confirm which mode the installer booted in. Finally, while there are any number of utilities to transfer Win7 Setup to USB for BIOS systems, a USB-based UEFI installation is also possible.
Note that you'll need to shrink the Win8 partition beforehand using Disk Management or 3rd party Windows partitioning software to free up space for Win7, and a defrag is recommended before shrinking for best results (or better still you can use GParted for this).
After Win7 is installed you'll need to boot from the Win8 DVD/USB and run Startup Repair or enter the BootRec /FixMbr, /FixBoot and /RebuildBcd parameters to restore the Win8 graphical bootloader and add a Win7 entry to it.