To start skinning, load up SkinStudio, choose New, and select a format - Universal Skin Format is a good choice. If you've used TGT Soft's StyleBuilder you'll notice many similar elements like the category shortcuts, tree list and preview areas.
Think about what you want to change, and then choose the appropriate third party editor. You can set it up by going to Tools | Preferences | Editing | Image editors | Add.
Now just select the element you'd like to change, double click on it to open it up in your default image editor, and modify how you like. After you've saved your modifications, the image will be updated in SkinStudio.
To complete your skin, just save it, ZIP it or, if you have WindowBlinds up and running, just apply it straight away.