I can't see the purpouse of putting iOS 7 to DFU mode at the moment. However the instructions are following:
1) Power the device down
2) Start holding Power button for 3 seconds
3) Hold Home button but don't let go of the Power button, 10 seconds
4) Keep holding Home and let go of Power. About 15 seconds just to be sure
That's it. You should now be in DFU mode.