By hand
You could do this using a gouge (a type of chisel with a curved cutting edge) but it would require that its edge be very sharp indeed* to get an acceptable surface inside the scoops. In addition to getting the gouge sharp enough initially this will require frequent upkeep of the edge during the job, as often as every few minutes depending on the hardness of the wood and how well it is cutting.
Router
Freehand routing with a round-nose bit might achieve this but it would require unerring attention on the part of the operator O_O
In either case some practice pieces will be required! Even with that I wouldn't expect to get a really nice result first time.
*Sharpening a gouge to an equivalent level to a straight-edged chisel is something that many find much more difficult, some guidance that will help achieve the necessary sharpness (there's really only one standard to aim for, hair-poppingly sharp) is given in a previous Answer, How do I sharpen curved tools like gouges?