Step 1. Download the Firmware and launch Sn0wbreeze, make sure you run as administrator (if running Vista or 7).
Step 2. Click on the Blue next arrow. At this point you get option for choosing between two modes: Simple and Expert. We will do simple for now, choosing Expert is totally upto your experience level.
Step 3. Click on Browse and locate the 3.1.2 firmware. At this point, if you make a Jailbreak for iPhone, the application will ask whether you want to activate iPhone. That’s simple, if you are on unofficial career, Select YES, else NO. and Click Start to start creating custom firmware and wait for several minutes.
Step 4. Put your phone into DFU mode using the on-screen instructions.
Step 5. Open iTunes and SHIFT +Restore and browse the custom firmware you just created. Wait for iTunes to finish off its stanard recovery procedure and you are done.