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 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.