public static void Sign( 
   string inputFileName,
   string outputFileName,
   DigitalCertificate certificate,
   SignOptions signOptions
)

// 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 );