Here are some simple steps I use to restore friends their iPhones.

1. Connect your iPhone to your computer.
2. Turn of your iPhone by holding the power button
3. Wait for your iPhone to completely shutdown
4. Hold POWER and HOME for 10 seconds
5. Release POWER but keep holding HOME
6. 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).