Reddinosaur- I start our making the center (2" stock, long enough to chuck it and allow for you to work on the desired depth of the ornament) and drilling a 1/2 hole and shape the the outside, sanding and finishing leave enough material to support it while using a small piece of tool steel in a handle to hollow out the inside of the ornament (This helps reduce the weight so that it does not pull on the trees limb) .
After you finish the hollowing, separate the center. I made a taper dowel from a pen blank that is slight larger than 1/2 inch down to 1/2 inch. Slide the center on to the dowel and do your touch up cuts and final sanding and finishing. The center is done. Then all you have to do is decide if you are going to put a cap on the top or a finial and make the top and bottom attachements fitting them to the 1/2" hole that you drilled.
You can make them out of one piece but will need to make them smaller or use lighter weighted wood.