A method for instructing a 3D printing system that includes a 3D printer provided with a printing coordinate system to print at least one first object onto an existing second object comprises providing or receiving at least one image representing at least a part of the existing second object, determining or receiving an alignment between at least part of the at least one first object and at least part of the existing second object, determining a pose of the existing second object relative to the printing coordinate system according to the at least one image, and providing the 3D printing system with the pose and the alignment for the 3D printer to print at least part of the at least one first object onto the existing second object according to the pose and the alignment.