Step 1: Take a piece of square section timber. You'll need to make sure that adjacent faces are cut/planed at 90 degrees to each other. 

Step 2: Then take another three identical pieces and temporarily fix them together. You may want to use a little PVA glue at the ends and then bolt them together in the waste area at each end. Remove the bolts when the glue is dry. You don't want protruding bolts whizzing around at high speed while you're trying to turn the workpiece. 

Step 3:  Mount the workpiece between centres.

Step 4: Turn a long, deep concave shape in the workpiece.

Here's a picture of the actual piece turned during the demonstration.

Step 5: Remove from lathe and separate the four pieces of wood. Reassemble them so that the outside concave shape you turned in Step 4 now becomes the inner shape. This is achieved by turning each piece through 180 degrees. Fix them together again securely - this time the joint is permanent. Use the oval shape in the centre as a reference point to make sure they're aligned correctly.

Step 6: Finish turning the outside surface of the piece. Add finials or whatever other details are required by the design. 

Here's an example of a piece made by Ian on an earlier occasion where he's added a bell in the centre and finished it off with decorative components on each end.