Need clarification with 06.15.00 bb on 4.2.1

[Kadin]

I'm on a 3GS right now with 4.2.1 and 06.15.00 without ultrasn0w set for the unlock. Unlock isn't important. But what's the process I need to go through right now to get up to the current iOS version?

From what I recall I think I need a modified .ipsw that doesn't change the bb, right? If anyone would be kind enough to maybe give me a step or two beyond this, that would be great... thanks!

 

[bwrairen]

use PwnageTool to build a custom IPSW

 

[Kadin]

use PwnageTool to build a custom IPSW

So I'm basically downloading the official 4.3.1 .ipsw and then loading up PwnageTool and then saying 'No' which will then give me a custom version?