1. A method for instructing a 3D printing system comprising a 3D printer having a printing coordinate system operable to print at least one first object onto an existing object, the method comprising: obtaining, by at least one camera, at least one image representing at least a part of an existing object; determining a camera pose of the at least one camera relative to the existing object when the at least one image is captured; determining an alignment between at least part of the existing object and at least part of a first object to be printed on the existing object based, at least in part, on the determined camera pose; determining a pose of the existing object relative to a printing coordinate system according to the at least one image; and directing a 3D printer to print at least part of the at least one first object onto the existing object according to the pose and the alignment.