Set the waves before the stitches
Mark or use the printed arcs clearly enough to see where one fan begins and the next continues. Check the edge crop before working the centre so the final row does not end at an accidental half-wave.
A small sample is especially useful on a plain cloth because it shows whether the fabric allows the curve to remain smooth.
Keep the arc rhythm consistent
Use short stitch groups around the top of each curve and let the thread settle before moving into the next line. A tight pull can flatten the arc and make the waves look like points.
Look across several repeats, not only at the curve under the needle. The pattern reads as a field, so an occasional small difference is less important than the overall spacing.
Choose a cloth that supports the movement
A stable cotton or prepared sashiko cloth is a straightforward starting point. Very loose or stretchy fabric can make the arcs wander unless it has been supported appropriately.
Use a thread weight that keeps the gaps visible. If the waves close together, change the pairing before trying to compensate by pulling harder.