Here are some simple steps I use to restore friends their iPhones.
- Connect your iPhone to your computer.
- Turn of your iPhone by holding the power button
- Wait for your iPhone to completely shutdown
- Hold POWER and HOME for 10 seconds
- Release POWER but keep holding HOME
- iTunes should now detect your iPhone in DFU mode
Note, in case you do a redsn0w jailbreak, you might need to disconnect press power and reconnect the iPhone to make it work (do this quickly).