Before we begin there are some things you need to know First of all for the iPhone 3GS, iPod Touch 3G, and the newer iPod Touch 2G(beginning with "MC") jailbreaking 3.1.3 is NOT POSSIBLE at the moment. Do not try. We strongly recommend you DO NOT upgrade to 3.1.3 if you use one of these devices.
Next order of business is for iPhone 3G users. DO NOT Upgrade to 3.1.3 or follow this guide if you need an unlock. Upgrading to 3.1.3 will upgrade your baseband which is not currently unockable.
Lets download the necessary files.
Download the 3.1.2 firmware for your device:
iPod Touch 1G & iPod Touch 2G users you'll need to find the 3.1.2 .IPSW that iTunes downloaded onto your computer when you originally updated to 3.1.2
Next Download redsn0w:
Windows redsn0w 0.9.4 download
OS X redsn0w 0.9.4 download
Now that we have everything downloaded connect your device to iTunes and allow it to download and update to 3.1.3.
Once the update via iTunes to 3.1.3 is done close out itunes and start up redsn0w.
Clck Browse, select the 3.1.2 firmware that you downloaded for your device, and Open.
redsn0w processes and identifies the firmware
click Next, redsnow will do some work
Select the options you want. Do not select Already pwned or Unlock. For this guide we're just installing Cydia. Click Next.
Make sure your device is turend off and plugged in, then click Next.
We have to get into DFU mode. Simple follow the prompts. First hold the power button for 3 seconds.
Continue holding the power button and press the home button. Hold both for ten seconds.
Release the power button but continue holding the home button for 30 seconds.
redsn0w works its magic
Once completed the redsn0w windows says Done! Click Finish and redsn0w exits. Things will still be going on on your device so don't mess with it until it has fully rebooted to the lockscreen. Congrats! You have jailbroken 3.1.3 for your Device!
Source: modmyi
