To clarify the request:
The step "Visual Basic => Tools => Digitial Signature"
have to be automated.

This is from the VBA help file.  However, it requires that a digital
signature certificate be in existence.

Example
The following example prompts the user to select a digital signature with
which to sign the active document in Microsoft Word. To use this example,
open a document in Word and pass this function the name of a certificate
issuer and the name of a certificate signer that match the Issued By and
Issued To fields of a digital certificate in the Digital Certificates dialog
box. This example will test to make sure that the digital signature that the
user selects meets certain criteria, such as not having expired, before the
new signature is committed to the disk.

Function AddSignature(ByVal strIssuer As String, _
   strSigner As String) As Boolean

   On Error GoTo Error_Handler

   Dim sig As Signature

   'Display the dialog box that lets the
   'user select a digital signature.
   'If the user selects a signature, then
   'it is added to the Signatures
   'collection. If the user doesn't, then
   'an error is returned.
   Set sig = ActiveDocument.Signatures.Add

   'Test several properties before commiting the Signature object to disk.
   If sig.Issuer = strIssuer And _
       sig.Signer = strSigner And _
       sig.IsCertificateExpired = False And _
       sig.IsCertificateRevoked = False And _
       sig.IsValid = True Then

       MsgBox "Signed"
       AddSignature = True
   'Otherwise, remove the Signature object from the SignatureSet collection.
   Else
       sig.Delete
       MsgBox "Not signed"
       AddSignature = False
   End If

   'Commit all signatures in the SignatureSet collection to the disk.
   ActiveDocument.Signatures.Commit

   Exit Function
Error_Handler:
   AddSignature = False
   MsgBox "Action cancelled."
End Function