I use mark out lines on a blank as described in Stuart Mortimer' book Techniques of Spiral Work. That book is out of print or very expensive to buy new or used today.
For open twist, drill center of blank with appropriate size drill bit. Will use small handsaw, dremel with saw blade, or drill bit to open the twist. Then it is just a lot of filing with rasp, files and final sanding.
On black potpourri vase, open finial easier to make because easier to sand, than vase. Can use any wood you like as long as free of knots or other defects.
Richard Madden put out a pretty good article:
How I Make Spiral or Twisted Finials
http://www.sawmillcreek.org/content.php?143-How-I-Make-Spiral-or-Twisted-Finials&
Think Woodturning Design did an article on Spiral turning, not sure what edition. You can also find articles at Goggle Images and Books.