How to align duplication correct way With the reference image attach. Im making the sandals and im the way to plug the diamonds in that. I have remove the center edge from the mesh. With the duplication im making the diamond to rotate around the curve. From side view it looks good but when i rotate and see the diamond going upper side and some diamond gone inside the mesh and only 1 diamond ring is visible. How correct align with the reference image the diamonds.
With the curve selected, under the Curve tab of the properties area, activate Follow.



Go into Edit Mode and change the curve's Tilt so that the curve's arrows are perpendicular to the shoe geometry. With a control point selected use Curve > Control Points > Tilt or Ctrl+T to do that. I suggest enabling Proportional Editing to achieve a smooth transition. (The tilt can also be changed after the diamonds are aligned, which probably will make errors more obvious.)



Change the diamonds' Rotation so that the diamonds are aligned with the surface.
XYZ angles of 0°, -15° and -90° seem to work fine.



To make sure that the curve is running at the surface of the shoe, add a Shrinkwrap Modifier with the shoe as target, mode being Nearest Surface Point and enable Apply on Spline.



You might have to move some of the curve's control points outwards so that the shrinkwrap modifier doesn't snap the diamonds to the inside of the shoe.



Final blend file: