If you have at least jailbroken, with Cydia, take a look at this.

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iPhone 3GS / 3.1.2 JB (PwnageTool) / 04.26.08 carrier-locked, Carrier Logo Fixer / Cydia / 1000 posts on Hackint0sh

Installing Cydia programs on a phone that has no internet connection: read this.

Editing binary .plist, .strings, .nib and .xib files:
* on your computer: Windows tool / conversion website.
* on your iPhone: convert those files in a terminal with plutil (installed with Erica Utilities) or edit them with iFile (Cydia links).

Anyway, you should have jailbroken iphone with Cydia already installed. iTunes should be installed on the PC. Download openssh*.deb file from one of the suitable repositories to your PC. Use BigBoss repo, for instance. The filename is
http://iphonebigboss.com/repo/repofi...honeos-arm.deb
Next you must transfer the file over USB cable from the PC to the iphone using DiskAid or something similar.
The next task is to let Cydia know about this new package.
I recommend this way to do
http://www.hackint0sh.org/forum/385516-post14.htm
i.e. to create a folder /private/var/root/Media/Cydia/AutoInstall/ and drop the .deb file there.

instead of installing that software, a more portable way is to fetch a copy of iphone_tunnel:
http://oss.coresecurity.com/projects/iphonedbg.html
start ssh on iphone, run iphone_tunnel 22 22 and ssh with winscp to localhost port 22

This can be portable too! All you actually need are winscp and iTunnel folders. First you run Launcher.exe in iTunnel, then winscp.exe. And of course, a batch file is a neat idea.

In order to make it work under OS3.0 you need to use iTunes 8.2 and replace iTunesMobileDevice.dll in directory where itunnel.exe is located with the new one from C:\Program Files\Common Files\Apple\Mobile Device Support\bin.