How to Jailbreak iPhone 3g on iOS 4.2 or 4.2.1 (untethered):
1. Download Redsn0w (Windows) here (Mac) here
2. Download the iPhone 3g 4.2 or 4.2.1 firmware here
3. When finished downloading, extract Redsn0w (if needed)
4. Open Redsn0w by, right clicking on it and running it as administrator (it is okay if it does not give you the option to "Run as Administrator," just proceed with opening it normally)
5. Once open, browse, and browse for your 4.2 or 4.2.1 firmware (.ipsw file)
6. Once your ipsw is verified click next
7. Let it do it's thing
8. Once on the select options screen select "Cydia" only (unless you wish otherwise) and proceed
9. Once on the next page, read explanation, and TURN OFF your device while still connected to your computer
10. Now be ready from here on everything is fast paced
11. BEFORE YOU PROCEED go over what will be on the next page which will be: A. hold power button for 3 seconds. B. Then while still holding the power button hold the home button and hold the combination for 10 seconds. C. Then release the power button while still holding on to the home button and hold the home button for approximately 30 seconds
12. Once you've gone over the procedure press next
13. hold the power button for 3 seconds
14. While still holding the power button hold the home button and hold the combination for 10 seconds
15. Release the power button while still holding on to the home button and hold down the home button for approximately 30 seconds
16. Give it 10-20 minutes do do it's thing
17. When done, go on the device, find the Cydia icon
18. And your done!