How do I boot my iPhone with a tethered jailbreak?
Once you have jailbroken your iPhone 4 or iPhone 3GS MC model with redsn0w you must boot the device tethered for Cydia to work and the device to operate in a jailbroken state. To boot your iPhone tethered follow these steps:
1. Plug your iPhone into your computer.
2. Turn off the iPhone.
3. Follow these instructions to put the iPhone in DFU mode.
4. Run the redsn0w program on your computer.
5. Navigate to Extras and select Just Boot.
6. Follow the on-screen instructions and your iPhone will boot jailbroken.