uses
 ComObj;
procedure TForm1.Button1Click(Sender: TObject);
const
 // Word Document to open
 YourWordDocument = 'c:\test\worddoc.doc';
var
 BookmarkName, Doc, R: OleVariant;
begin
 // Start a Word instance
 try
  WordApp := CreateOleObject('Word.Application');
 except
  ShowMessage('Could not start MS Word!');
 end;
 // Open your Word document
 WordApp.Documents.Open(YourWordDocument);
 Doc := WordApp.ActiveDocument;
 // name of your bookmark
 BookmarkName := 'MyBookMark';
 // Check if bookmark exists
 if Doc.Bookmarks.Exists(BookmarkName) then
 begin
  R := Doc.Bookmarks.Item(BookmarkName).Range;
  // Add text at our bookmark
  R.InsertAfter('Text in bookmark');
  // You make a text formatting like changing its color
  R.Font.Color := clRed;
 end;
 // Save your document and quit Word
 if not VarIsEmpty(WordApp) then
 begin
  WordApp.DisplayAlerts := 0;
  WordApp.Documents.Item(1).Save;
  WordApp.Quit;
  BookmarkName := Unassigned;
  R := Unassigned;
  WordApp := Unassigned;
 end;
end;