Swami's rope trick
Curved walls are simple with this mystic rope trick handed down from the ages.
Ropes are put around the wall about every 5 feet.
Nails are bent over the rope to make them easy to remove later.
The wall is then filled in between the ropes and rodded off with a 6 foot rod.
We cut grooves along the bottom and the top of the ropes and pulled them out.
The grooves are filled in and the wall is ready for the finish coat. The stair tower is nice and round and straight.
The brown coat on the stair tower is as straight and round as you want.
This is a slick trick for round walls and barrel ceilings. For smaller curves, and more precise applications, a running mold or a form template must be made.
Another good method is to use an old hose. Most plasterers have an old useless hose lying around. For inside plaster, you can even leave the hose in the mortar and it won't crack.