Ok. Here's How
Download or create the theme.
Place it all in a folder named for the theme. In that folder should be the following (if you plan on keeping any of these stock, you can leave them out): Background.png, Dock.png, StatusBar.png, and a folder named Icons with .png files that are 60x60 for every icon you want to replace (which should each have the same name as the application you are replacing, as they appear on the SprinBoard labels)
Download Cyber Duck (or another FTP client)
Find out you iPod's IP address by going into settings and pressing WiFi then pressing the blue arrow next to the network you are on.
Now in your FTP client, start an SFTP connection to the IP address you got from your iPod. Make sure it is SFTP or it will not work. The username is root and the password is alpine.
That should bring you up to a screen with two folders and a file. Library, Media and the file I cannot remember.
Open the Library folder, and then open the SummerBoard folder.
Place your theme into the folder you find there called Themes.
Open SummerBoard and your theme should be there
Fairly simple to do when you do it once or twice you will be able to do it without thinking
If you have any questions feel free to ask