Turning a ball is more work than you think.
I first tried a simple method..
Turn a blank round
Form the ball
Part off and sand.
Clearly, not a ball...
On to step two!
Chuck up a chunk of walnut, and scrape out a cup...
Part off for the tailstock....
Scrape out an matched set for the chuck side..
They should match up.
Insert square block..
Okay. You turn off the edges, rotate your block. Turn off more edges, rotate your block.
Etc. About 15-20 minutes later...
You can see how I turned a way the jam chuck as I went. The closer you get to spherical, the less you remove each rotation. Like anything practice till you're happy with it!