Copy/move the downloaded file to the skin file in bootskins. I believe the default file is "C:\Program Files\Stardock\WinCustomize\BootSkin\skins"
Open the bootskin program. At the top there there is "file" and "help" button. Click on the file button. The drop down menu gives you five choices. The top one "import from file" is the one you want to click on. Once clicked it should open the default location of the skin file. If not just browse to you get to it. Once you find it click on the new download and click on open. That's it! you should get a message that it was installed sucessfully. You can preview and/or use it now. You'll notice that when you do that...the skin folder now has the bootskin file unzipped and in its own folder.
Hope that helps. Occasionally I will get a folder that Bootskin cannot unzip. Either try downloading it again(sometimes it is a bad download), or the file was incomplete. Try a different file then.