JB/Unlock Iphone 3g 3.1.2 +Upgrading firmware

[MBR12]

I have an iphone 3g 3.1.2 that I need to jb and unlock, however I would like to upgrade while preserving my 5.11.07 baseband. I've seen a million tutorials, but I'm confused...do I upload a custom ipsw first?

Thanks!

 

[eastercat]

Yes you use a custom ipsw.
Press <option> and restore in iTunes to pick the file.