It's easy.
You need just a couple of things.
1. The IP address of your Xbox. If you are using Evox or something similar, it's easy to find. Let me know if you can't find it.
2. An FTP program. You can do it with IE but it sucks. Most people recommend Flashfxp. Download it here:
http://www.flashfxp.com/Download and install it. You get a 30 day trial or something with it. Once its installed and ran, look on the toolbar in the middle for the "yellow" lightning bolt. Click it and "Quick connect". In the window that pops up, type your IP address for the XBox in the Server or URL space, user name in that box, and then the password in the appropriate box.
You're in.

Drag the files you want to upload from the sub-window on the left to the window at the bottom. In the right window, move in the directory to the folder that you want to upload to, then hit the 3rd button from the right (labeled Transfer selected" and BOOM, its done.
You may search the Xbox forums if you want pictures. I read one a few years ago that had some good info. I'll search for it and post if/when I find it.
It's pretty easy to learn. Good luck.
