Help create my XML tree

Hi, I have a problem with creating an XML tree, I'm starting but I would like you to paint this line at the beginning "" "" "", if you can also close it but only read the "UBLExtensions" but not the ext: and when I put it as a string variable to paint everything, I get an error that can not paint the characters. Someone who has a PDF or someone has the solution to this?

  //ENCABEZADO DEL XML
            string linea1 = "<ext:UBLExtensions>" +
                            "<ext:UBLExtension>" +
                            "<ext:ExtensionContent>" +
                            "<ds:Signature Id=\"signatureKG\">" +
                            "<ds:SignedInfo>" +
                            "<ds:CanonicalizationMethod Algorithm=\"http://www.w3.org/TR/2001/REC-xml-c14n20010315#WithComments\"/>" +
                            "<ds:SignatureMethod Algorithm=\"http://www.w3.org/2000/09/xmldsig#dsa-sha1\"/>" +
                            "<ds:Reference URI=\"\">" +
                            "<ds:Transforms>" +
                            "<ds:Transform Algorithm=\"http://www.w3.org/2000/09/xmldsig#envelopedsignature\"/>" +
                            "</ds:Transforms>" +
                            "<ds:DigestMethod Algorithm=\"http://www.w3.org/2000/09/xmldsig#sha1\"/>" +
                            "<ds:DigestValue>+pruib33lOapq6GSw58GgQLR8VGIGqANloj4EqB1cb4=</ds:DigestValue>" +
                            "</ds:Reference>" +
                            "</ds:SignedInfo>" +
                            "<ds:SignatureValue> AQUI VA LA SIGNATURA</ ds:SignatureValue > " +
                            "<ds:KeyInfo>" +
                            "<ds:X509Data>" +
                            "<ds:X509Certificate> FIRMA </ds:X509Certificate>" +
                            "</ds:X509Data>" +
                            "</ds:KeyInfo>" +
                            "</ds:Signature>" +
                            "</ext:ExtensionContent>" +
                            "</ext:UBLExtension>" + 
                            "</ext:UBLExtensions>";

            XmlDocument doc = new XmlDocument();
            XmlElement raiz = doc.CreateElement("ext",":","UBLExtension");


            doc.Save("D:\CodeDesign\archivo.xml");

I would like the result to come out like this.

  "<ext:UBLExtensions>" +
       "<ext:UBLExtension>" +
       "<ext:ExtensionContent>" +
           "<ds:Signature Id=\"signatureKG\">" +
               "<ds:SignedInfo>" +
                  "<ds:CanonicalizationMethod Algorithm=\"http://www.w3.org/TR/2001/REC-xml-c14n20010315#WithComments\"/>" +
                        "<ds:SignatureMethod Algorithm=\"http://www.w3.org/2000/09/xmldsig#dsa-sha1\"/>" +
                        "<ds:Reference URI=\"\">" +
                        "<ds:Transforms>" +
                        "<ds:Transform Algorithm=\"http://www.w3.org/2000/09/xmldsig#envelopedsignature\"/>" +
                        "</ds:Transforms>" +
                        "<ds:DigestMethod Algorithm=\"http://www.w3.org/2000/09/xmldsig#sha1\"/>" +
                        "<ds:DigestValue>+pruib33lOapq6GSw58GgQLR8VGIGqANloj4EqB1cb4=</ds:DigestValue>" +
                        "</ds:Reference>" +
                        "</ds:SignedInfo>" +
                        "<ds:SignatureValue> AQUI VA LA SIGNATURA</ ds:SignatureValue > " +
             "<ds:KeyInfo>" +
           "<ds:X509Data>" +
         "<ds:X509Certificate> FIRMA </ds:X509Certificate>" +
      "</ds:X509Data>" +
     "</ds:KeyInfo>" +
   "</ds:Signature>" +
 "</ext:ExtensionContent>"