Start necking in from the place where you want the form to start
tapering in. Raise as usual.
My favorite book on hollowware is by Feingold and Seitz. There is a
chapter on raising in the book I co-wrote with Betty Helen Longhi,
Creative Metal Forming. We have a group on Facebook called Creative
Metal Forming that you might enjoy being part of. Steve Shelby posts
photos of his works in progress there, and on his website. You also
might like to take a look at David Huang’s website----he documents
many of his pieces, and shares the photos.