This is what I come up:

        ' Add a stylesheet
        ' expC1 Url
        Public Function AddStylesheet(ByVal tcUrl As String) As Boolean
            Dim lcText As String = ""
            Dim lcUrl As String = ""
            Dim llSuccess As Boolean = False
            Dim loMemoryStream As MemoryStream = New MemoryStream()
            Dim loXmlDocument As XmlDocument = New XmlDocument()
            Dim loXmlProcessingInstruction As XmlProcessingInstruction = Nothing
            Dim loXmlTextWriter As XmlTextWriter = Nothing

            ' Reset everything
            cMessage = ""

            ' Initialization
            lcUrl = tcUrl

            ' Trim everything
            lcUrl = Trim(lcUrl)

            Try

                ' Now load the in-memory stream XML data into the XML document
                loMemoryStream.Seek(0, SeekOrigin.Begin)
                loXmlDocument.Load(loMemoryStream)

                ' Now add the stylesheet processing instruction to the XML document
                lcText = "type=""text/xsl"" href=""" + lcUrl + """"
                loXmlProcessingInstruction = loXmlDocument.CreateProcessingInstruction("xml-stylesheet", lcText)

                loXmlDocument.InsertAfter(loXmlProcessingInstruction, loXmlDocument.FirstChild)

                ' Now write the document out to the final output stream
                loXmlTextWriter = New XmlTextWriter(oMemoryStream, System.Text.Encoding.ASCII)
                loXmlDocument.WriteTo(loXmlTextWriter)
                loXmlTextWriter.Flush()

                llSuccess = True
            Catch loError As Exception
                cMessage = loError.Message
            End Try

            Return llSuccess
        End Function

oMemoryStream already exists as part of the class and is initialized when this method is called. This is pretty much as simplified as I could see unless there is something new since the last six years which would be better.