The process is actually quite simple -- with the complicated bit at the end...
You need to boot from some kind of Linux LiveCD, or from a GParted LiveCD to do the partitioning -- because you can't modify a partition that is mounted and you can run using a partition unless it is mounted. Don't know what the Zorin media has, but the Ubuntu desktop CDs are bootable, and when you boot from them, choose Try Ubuntu. Once in there use the Disk Utility to shrink the partitions to leave some room.
Do NOT use that utility to create a new partition. While running GParted under Windows will probably let you create an NTFS partition and assign it a drive letter, it would probably be better to have the formatting done as a side-effect of the Windows 7 installation.
Since Windows 7 will overwrite the MBR as a side-effect of its installation, when it is done, you will no longer have access to Zorin. If Zorin used GRUB, you will then need to reinstall GRUB to regain access. You would need to check on the Zorin forums (if they exist) or on the Ubuntu forums for how to reinstall GRUB. For Ubuntu, you need a desktop CD because you reinstall GRUB using it.
However, once you reinstall GRUB, if you install it to the Drive, not to a partition, you will NOW no longer have access to Windows 7. In Ubuntu, you would then open a terminal and enter "sudo update-grub" -- to regenerate the GRUB menu so it includes Windows 7. I have no idea how you would do this in Zorin.