@iori_argami
Make sure u download the correct firmware for the 1st gen iphone. I suggest to stay with 2.0.1 since the 1st gen don't benefit much from fw2.0.2. U can download it here.

When u finish downloading it, restore the phone with the stock fw (not the custom one). This should settle the error 6 that u been having. Once its finished restoring (u'll be at the emergency call only display), close itunes and start with the winpwn process to create custom fw. Make sure the 2 icon on the top right shows Custom and Expert:On. We will use the expert mode to be more detail.

1. Choose the corresponding iphone
2. Choose the corresponding fw
3. Choose not to unlock the phone. Do it later thru Cydia
4. Choose to activate iphone
5. Choose to activate YouTube
6. Choose to install Cydia (mandatory)
7. It's up to u if u want to install installer 4 as its still very buggy.
8. Choose to resize partition
9. Do not wipe data
10. If you want to use custom boot logo then go for thumbs up. If not then the opposite
11. Increase the root partition size to 600mb
12. Search for the fw that u just downloaded
13. Select the place where u want to save the custom fw
14. It will create the custom firmware
15. Follow the instruction to put the phone into DFU (DFU mode means the screen is black with nothing on it, itunes will detect it and ask to restore)
16. Restore the phone with custom fw while in DFU mode

After restore, open Cydia n find bootneuter and use it to unlock/flash back the baseband. That should cover the steps ..... I think.