Step 1: erase the hard disk with DBAN (free download)
Step 2: install windows 7 from the installation disk
Step 3: install drivers and utility programs that Dell offers for this machine for Windows 7

I'd add a step 0: make a clone of the drive so you can go back to Windows 8 later. But that's up to you.

However, I wonder why you didn't simply a Dell 15z with Windows 7. It's the only one they offer at http://www.dell.com/us/business/p/xps-15z/pd So before doing all of this I'd contact Dell to see if they can sell you recovery disks for Windows 7. Or maybe you're still inside a 30-day return period? That would be the easiest.

Kees