Hello again
As I mentioned before, I have an at&t locked 3gs 3.1.2

I think it's on the old bootrom as my serial number's 4th and 5th digits are 36

I think I will need to use redsn0w to jailbreak and then ultrasn0w to unlock the phone, keeping my 3.1.2 firmware.
jailbreakme.com seems simpler but it's only for 4.x firmware, right?

I want to get this absolutely right so I need to bug you again!

I can't find any guide that will take me through all of the steps required without missing a bit

My current doubt is about the sim card

I have used a phonebook sim to get past the emergency screen. it worked.
I am too afraid to remove it at the wrong time ( I have read on iclarified that now phonebook cards work just once..)

My question is:

at what exact point of the activation, jaibreak, unlock procedure do I need to remove the phonebook sim?

Also, I suppose the new carrier sim will need to go in at the very end, right?

Should I mind other steps to make sure at the end of all this I have a properly activated , jailbreaked and unlocked machine?