Some steps in this tutorial meant to help you to upgrade from 2.x / 3.0.x OS to 3.1.2 OS, please ignore them if you have already OS 3

1- First u have to download & install iTunes in your computer

from here (You dont need to do this if you  already have OS 3.0 installed on your IPHONE)

2- Download Blackra1n from here

3- Download OS 3.1.2

(2G): iPhone1,1_3.1.2_7D11_Restore.ipsw

(3G): iPhone1,2_3.1.2_7D11_Restore.ipsw

(3GS): iPhone2,1_3.1.2_7D11_Restore.ipsw

to your computer, make sure to remember where did you save it.

4- Connect your iPhone to the computer & run the iTunes (You dont need to do this if you  already have OS 3.1.2 installed on your IPHONE)

5- Press & hold the Shift Button in your keyboard & press the restore button in the iTunes (You dont need to do this if you  already have OS 3.0 installed on your IPHONE)

7- it will ask u for the OS 3.1.2 file just navigate for it & select it (You dont need to do this if you  already have OS 3.0 installed on your IPHONE)

wait for the iTunes to finish installing the new firmware, once its finish

PLEASE READ CAREFULLY SEVERAL TIMES BEFORE YOU START THE PROCESS:

8- double click the blackra1n & run the program

9- Make sure your iPhone is connected to the computer then click the make it ra1n button

10- Your iPhone will now be placed in recovery mode.

11- You will see a picture of GeoHot that replaces the regular recovery mode screen on your device.

12- blackra1n will run and then your device will reboot.

13- A popup message will appear informing you that BlackRa1n is free software and if you appreciate it you can donate

14- Once your device reboots it will be jailbroken and you will notice a blackra1n icon on the Springboard. Make sure you have an internet connection and press to launch the application.

15- Choose the installer application of your choice then press to select sn0w if you would like to unlock your device. Press the Install button at the top right of the screen to begin.

16- BlackRa1n will begin to download and install the package you selected.

17- If you chose to unlock device using blacksn0w you will be presented with a Post Install log for sn0w. CommCenter will be restarted and there is no need to reboot.

18- Press the Close button to continue.

19- Once complete BlackRa1n will respring your iPhone and you will find the installer package of your choice on the Springboard.

20- Once you have confirmed your installer package is working correctly you may use the Uninstall blackra1n button inside the BlackRa1n app to remove it from your springboard.

done