The first thing to do is finish the content of the document, and I mean finish.  The reason I stress this is that the next step can take a long time, and if you significantly modify the document (or help file or book) after your index is started or completed, then you will have to do it all over again.  Literally.  This can take hours, days, weeks or months, depending on the size of the document.  So you really, really don't want to do it twice.