Hi Mario - you know what - you should open the _default/skins/dnn-blue skin folder - in there is a perfect example of how to package the skin and the files that go with it.
It has a skin.xml example for you, and a skin.css file to look at and the html file, but more than that - the xml file has some references and patterns to follow to make changes.
The menu however is more tricky - and you might get a bit more insight by downloading one or more of my free skins and looking at both the ascx file and the css to see how the menu may require some inline code - eg.. code in the skin file or, the xml file if you're up to it, to translate the functions of the menu.
You see, things like spacers and separators, can't all be managed by css, so there are snippets of code you have in your skin file - if using ascx method, or, you will need some xml experience to make it happen using the html / xml method of skinning.
If you have the wrong format or attributes,they will not be recognised, for example - if something needs to be true or false, or a colour, or a number - you need to adhere to the xml rules to get it to work.
by the way - I also have the skinning documentation in a chm (help file) format for free download that has alot of these code references in them. They are also available in the documents folder in your dotnetnuke build (I'm pretty sure that's where they are now)
Nina Meiers