There is no Tutorial because no one in their right mind would upgrade the baseband if they had a 5.12 baseband. Most would go to great lengths to upgrade the firmware and keep that baseband for unlocking.
If you want to upgrade the baseband, just hook your phone up to your PC, open Itunes and it will inform you there is an update firmware available for your phone, if you click upgrade it to the latest firmware, your baseband will upgrade as well.
Please note that once you upgrade the baseband, you won't be able to unlock it with ultrasn0w if you decide to unlock your phone.
From there make sure you preserve your baseband (by running Redsnow) and can update using a custom FW IOS 5 tethered jalbreak (if you wan to move to IOS5 that is) with ultrasnow unlock.
IOS 4.1 is pretty stable right now and if you want an untethered JB that is your best bet.
Hope this helps.
To the rest:
Please stop posting useless and unhelpful posts.
Stuff like no one in their right mind will upgrade a blah blah comes across as if you have a chip on your shoulder. If you are going to unlock the phone you need to have specific Basebands. the 5.12.04 isn't unlockable and therefore the OP has to move up to 5.13.04 in order to unlock the phone.
Chill and just try to help the OP out.
Everyone has a right to ask questions; dont make them feel sorry for it.