If i already have BlackSn0w installed, do i just insert the SIM card and let it get a signal or do i have to power off or reinstall blacksn0w?
If you have Blacksnow your phone is Jailbroken not unlocked so if the SIM belongs to the network the phone is locked to put it in and bam away you go. I always power my phone off when I put the SIM in or out. If the SIM is not for the carrier your phone is locked to it will not work you need to unlock it for it to work.