So, I got as far as to jailbreak and unlock w/ the 6.15 BB. I went to install Ultrasn0w 1.2 and now it hangs at boot up. Every five minutes or so it flashes and restarts the boot process. I can get it to boot into tethered mode, but I'm not sure where to go from this point.
Well you want to restore. I have a feeling you probably won't get very far with restoring to stock 4.2.1 as you'll just get stuck in recovery mode.
I recommend you use PwnageTool to create a custom 4.2.1 fw without the baseband update (since you've already done that part) and is activated if you do not have AT&T. If you don't have a Mac, you can find some of the members here on MMi can help. iYeow and eyeRobot have custom firmwares that they may help you with.
After obtaining/building the custom firmware, if you're using PwnageTool just allow it to set your phone into DFU mode.
If you have PC, get Redsn0w 0.9.6b6 (you'll need the stock 4.2.1 ipsw to run redsn0w) and allow it to set your phone into pwned DFU mode. Do not ask it to install Cydia or anything except pwned DFU.
Then you're free to shift+restore (option+restore on Mac) to the custom firmware.
So, I ran into a bit of trouble making my custom firmware. Pwnage Tool 4.1.3 will not allow me to make custom firmware from a 4.2.1 IPSW. Am I doing something wrong?
When I go to add the 4.2.1 firmware to Pwnage tool it says "Wrong firmware bundle". I'm using the Unlock Edition... not sure if 4.1.2 version of Pwnage Tool would work better.
I've already downloaded another copy of the IPSW with no luck.
Might consider finding iYeow and personally asking him. He knows more about making custom firmwares than I do. He makes them so I think he can help you solve this.
The reason why pwnage tool does not recognize the 4.2.1 bundle is because you did not read the instructions. You need to make a mod to the tool before using it.
You do this by right mouse clicking on pwnage tool and going to contents - resources - firmware bundles and drop the bundle that you get from DEV Team blog update #1 here Dev-Team Blog
That will enable pwnage tool to recognize your ipsw.