Before you activate your iPhone 3GS without a sim card, otherwise known as hacktivating, you need to make sure you have the following applications downloaded:
IPSW version 6.0 instead of 6.1.3 or any other versions.
After you download RedsnOw, you will be able to install Cydia or click on “Activate Device” whichever shows up during your RedsnOw download. Then, with RedsnOw, IPSW version 6.0 and Cydia (Or have clicked on Activate Device) is operational on your iPhone, you can move on to the hacktivation steps.
Steps to Hacktivate Your iPhone 3GS
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Make sure iPhone is connected to your computer via your USB cord.
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Hold power button on your iPhone and slide to power off.
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Open RedsnOw (If you are using a Windows computer, run RedsnOw as Administrator).
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Choose Extras>Select IPSW and choose the 6.0 version.
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If a box pops up, choose “Yes” (Prompt may or may not show up on your iPhone).
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Choose Back and then Jailbreak, make sure install Cydia option is checked.
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Click on “Yes” to proceed.
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Follow DFU Instructions.
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When the process is complete you can close RedsnOw and complete the iPhone and iTunes activation steps.
Resource: Jailbreak QA