I'm having problems putting my iPhone 3G into DFU mode
To enter DFU mode:


Make sure the phone is on and iOS fully loaded, and iTunes is closed.
Plug the phone into your computer using the Apple proprietary cable.
Once plugged in, turn your phone off by holding the power button.
When the phone is completely powered down, perform the following
sequence:


Hold the power button for 3 secs.
Without letting go of the power button, press and hold the home
button for 10 secs.
Without letting go of the home button, release the power button for
another 10 secs.



Ignore what's happening on your screen, just time the sequence. If you are not succeeding, then you are not timing the sequence properly. There is only one way to enter the device into DFU mode. The one provided by Redsn0w is the correct one.