public class DocX : Xceed.Document.NET.Document 

// Add the SignatureLine to the document.
mySignatureLine = document.AddSignatureLine();

// Insert the SignatureLine in a document's paragraph.
myParagraph.AppendSignatureLine( mySignatureLine );

// Create a digital certificate in order to sign the document, by using a pfx file and its password.
var certificate = DigitalCertificate.Create( "CustomCertificate.pfx", "xceed" );

// Create SignOptions for the SignatureLine.
// Use the SignatureLineId to match the SignatureLine id and set the image used to sign the SignatureLine.
var signOptions = new SignOptions()
{
  SignatureLineId = mySignatureLine.Id,
  SignatureLineImage = "MarkStoneSignature.png",
  Comments = "This document is now signed by Mark Stone."
};   

// Sign the document with the certificate and the SignOptions, related to the SignatureLine.
DocX.Sign( "DocumentToSign.docx", "SignedDocument.docx", certificate, signOptions );