How to downgrade from ios 6.1 to 4.1 iphone 3gs?

Please do step by step instruction thanks

Added (1). To be able to install ipad baseband 6.15

Y in the world do you want to downgrade p.s please answer my latest question

Not going to be possible. Apple no longer signs for firmwares, lower than a certain point, iOS 6.0 is likely so 4.1 certainly would be out of the question. In any case, why would you want to downgrade & that far back?

Just download the 4.1 stock ipsw, go to iTunes and hit shift+restore and select the 4.1 ipsw.

Apple still signs 4.1 for iPhone 3GS so you don't need to do anything special with that. I'm assuming you want to install the ipad baseband to unlock the phone. You can do so with the newest version of Redsn0w and the stock 4.1 ipsw. After you do the baseband upgrade, go ahead and downgrade the baseband back down to 5.13.04 with Redsn0w. It's better to have the downgraded baseband. You can't skip the upgrade step and go straight to downgrade, so do the upgrade to the ipad baseband then downgrade it. Then you can jailbreak and unlock it.

(Note: You can take it back up to 6.1 if you want after you do the baseband flash. You'll just need to use a custom firmware that Redsn0w can make with a stock 6.1 ipsw.It'll let you restore to a 6.1 ipsw that does not have a baseband so that it won't overwrite the work you've done previously and keep the 5.13.04 ipsw. Then jb it with evasi0n, as the custom redsn0w ipsw will not include cydia).