From FreeCAD Documentation
- Make sure you have a mesh object. This can be created by the
Mesh Workbench, or by importing a mesh file type, like STL, OBJ, or DAE. If a solid object or solid file type (STEP) is used, it can be converted to a mesh using 
Mesh FromPartShape.
- Press
Curve on mesh.
- Press Start.
- Using the mouse pointer, pick points on the surface of the mesh in the 3D view; pick as many points as necessary to draw a smooth preview line.
- When enough points have been added, right click on the 3D view to open the context menu, and select Create. Depending on how smooth the original mesh is, one spline or multiple splines will be created in the tree view.
- Repeat the sequence Start → Pick → Create, to add more approximated splines.
- The new spline will be created, and will appear in the tree view, immediately after choosing Create; the task panel will remain active.
- Press Close to close the task panel, and terminate the command completely.
