had a job a few years back where the client wanted quite a big curve to his ceilings,so we bonded them out and formed the shape using cove trowels.used the trowel skimming too,but was finished with skimming trowel..
if you just want a smaller curve,you can use a bit of 4in waste pipe or even a glass bottle ( we call them curves a "bottle cove"down ere ) to form a shape ,but think you'll always find the last trowling up is with your skimming trowol anyway...