Here's how to fix this:
Enter DFU Mode.
You can do this by:
1) Hold down home button and the power button at the same time
2) When screen turns black, wait 1 second, then take off your finger from the power button BUT KEEP THE HOME BUTTON PRESSED DOWN.
3) You should see the connect to iTunes screen. If you didn't do it right, it will restart the phone as normal. Just try again and try to get the timing right.
Plug the phone into iTunes, open iTunes, then hold down Option while pressing the "Restore" button. It'll open a File Browser window, select the IPSW you downloaded, then it should work...
Let me know if you need more help/tips.
...And next time, DON'T DO THIS IF YOU'RE NOT A DEVELOPER!