How to jailbreak iPhone 3G/3Gs iOS 4.2.1 with Redsn0w/redsnow 0.9.6rc8 (0.9.6b5).

In addition, extract it in one folder. Now the next thing is we are going to connect out iPhone to computer and lunch iTunes. When you connect your iPhone to computer then select your phone from iTunes device list and then hold Shift and click on restore button.
We preferred restore option because it would not waste space.

So what we are gone do is first download the following files

Download...
Redsn0w 0.9.6rc8
Latest firmware 4.5.4

Here are some important steps, like go to that folder where you had extract the redsnow and firmware file, and select 4.2.1 firmware IPSW file then click on continue. Now wait for iTunes to finish your updates, after that launch your redsnow application from same folder. After opening this redsn0w application, click on browser button and select 4.2.1 IPSW file.

Let the software verify your firmware the click on continue to jailbreak. Now windows will prompt up. There you will select you required options (please make sure Cydia is select. For this tutorial, we will also select Custom boot logo and Custom recovery logo.). Here window will prompt up for selecting custom image logo file. Now click next to continue.

Now off your iPhone, plug it into computer, and click on Next. Now redsnow will ask you to turn you device into DFU mode. Put it to DFU mode. Hold down both the Home button and the Power button for 10 seconds. Release the Power button and continue holding the Home button until RedSn0w detects the device.

Now the redsnow will do the rest work wait until the phone gets restart.