Manually construct the result XML in web service?

It was a simple web method like this:

[WebMethod]
public DataSet test() {
  DataSet ds = new DataSet();
  ds ...;
  return ds;
}

Now I need more control over the result. So I'm trying to use XMLTextWriter to manually construct the result. But all the examples are reading/writing an XML file and XMLTextWriter also requires an System.IO.Stream ... How do I use it in web service? What is the return type if I manually construct an XML? How to redirect the IO stream to the return ... Hope I explained my point? Thanks for help.