1) Download the iPSW file for your device model from here: Http://theiphonewiki.com/wiki/Firmware. Make sure it's the rite one.(it will be iOS 7.0.3, unless you have SHSH blobs and want to restore to the same "older" iOS. This is a different process and only works on iPhone 4)
Save it to your desktop.
Properly enter DFU. YouTube it if your not 100% sure about how to do it.
Open iTunes.....wait for iTunes to recognize device in Recovery Mode. Click OK.
Hold down the "shift/ctrl" key on your keyboard *while* you click the restore button.
A window will pop up.
Navigate to your desktop and double click on the 7.0.3 iPSW file.