The easiest way to handle this is to acquire the WSDL files from the
webservice you are going to call. You then can use the .NET tools
WSDL.exe to create proxy classes to interface with the webservice. It
will handle creating the SOAP envelope for you.

If you do not want to use the proxy classes for whatever reason. You
should create serializable classes and use the XMLSerializer and/or
SOAPFormatter classes to serialize/deserialize the data.

Avoid concatenating strings together to create XML.