There is no magic ingredient required and you don't need to use Office automation to create XML documents.
Here's an intro to XML in Excel[
^] (you should understand what it is you will end up with).
All that is required is that you have a valid, conforming XML file. The .NET framework will write this out for you from an object model (look for XML serialization in the .NET framework if you're not familiar with the topic).
Hope this steers you in the right direction.
Cheers.