Hello,
Its just a matter of going to your style manager, pressing the drop down button on your style, and clicking export. Once the next page comes up, you don't need to change anything, just click download.
You need to download the image folder from your server (some style designers have it in /images/stylename/* where as I have it /styles/stylename/*) and include it with the style XML you just download. Simply Zip the two together and you have a simple vBulletin style package.
Hope this helps