Here is a very simple example.
System.Xml.XmlDocument d = new System.Xml.XmlDocument();
d.LoadXml(@"<animals><dog name=""Ruff"" /><cat name=""Nip"" /></animals>");
string str = d.OuterXml;
Save that string to the database as you would any other string. Make sure to use a data type that can hold a long string (if your XML has the potential to be large).