How do I put my iPhone 3G / original iPhone (2G) into DFU / recovery mode?

To put your iPhone into recovery (DFU) mode:

1. Make sure the iPhone is plugged into your computer.
2. Turn the iPhone off.
3. After the iPhone shuts down hold down the sleep button for three seconds.
4. While continuing to hold down the sleep button press and hold the home button.
5. Keep both buttons pressed for 10 seconds.
6. Release the sleep button but keep the home button pressed.
7. After a few seconds iTunes will say it has detected an iPhone in recovery mode.