Unlocking iPhone 4: how does it work?

I recently bought a preowned iPhone because I had mine stolen. However, the one I bought is with Rogers and I need to use it with Telus. I'm not sure I understand how unlocking works so I'm having a bit of trouble. All I know is that I'm too broke to pay 50$ to have it unlocked so I want to do it on my own. I have the IOS 6.0.1. If I jailbreak it, install Cydia and unlock it using ultrasn0w, is the only left to do is change the SIM card for a Telus one?

Added (1). If I order an unlock code online, do I have to change the SIM card or it's really just the code and I'm good to go?

Yea you can do it that way but that's altering the software - if you want a clean IMEI unlock, there are websites out there can do it cheaper than 50 - releasemycode.com is a great site that is pretty cheap to unlock

Actually some of the others who answered are wrong. You can't find a factory unlock provider who will charge under $100 to unlock a Rogers iPhone (except maybe Rogers themselves). If you ever do find one please let me know because I'm in the unlocking business! To answer your question, you can jailbreak and then use Ultrasnow to unlock it, but this is not permanent and your iOS updates form Apple are affected unlike a factory unlock. There are no codes when you use jailbreak + unlock.