if you right click anywhere on a dock you have up you should see an option that says "package image for sharing"
Actually, you would right-click, then select "This Dock", then "package images for sharing." Even easier, though is to click on the "configure dock" icon, and go here:
After navigating to the "themes" tab and choosing "package files," then you should see this screen:
Simply name your skin (whether it be icons or dock backgrounds, etc.) and add your files. When you press the "create package!" button, it will automatically generate a dockzip file.
Good luck! 
