Bob LaCivita replies: To make curved hand railings for a spiral stair requires a lot of clamps and money. If you are using a stock profile, you can buy pre-milled laminates from any good lumber yard. Most stair part companies sell this product. You are limited by the product you choose.
If you plan to do this with your own custom profile, determine your profile in a drawing. Slice the profile into about 1/8˝ segments. From here you have a series of knives ground for a shaper, molder or custom router bit maker. I would suggest putting a reference joint in as well, like a tongue and groove. You then saw your stock into the sizes needed for each shape. Profile each piece and make sure they fit together flat and straight.
Now you must build a form to match the stair run and rise of the stair, or you can use the stair and build a form on the treads. If the stair is not open or spaced at the landings, this can complicate matters. You then spread glue on the appropriate surfaces and clamp the whole mess to the form. You will need glue with a long open time. You unclamp the piece after 24 hours, You then scrape, plane and sand away the glue.
This is a very basic step answer. I could write a long article on the complexities of your question. Please note that this is not a project for a beginner or even an intermediate. Circular stair work is very complex. You must check all state and local building codes. When houses are inspected, the stairs are where the most building violations occur. You would hate to have to remake this piece. I hope this is caution enough.