In the case of a box, the necessary 3D wireframe model is created using my Esko software. The die-cut drawing present in the artwork can be easily converted into a 3D model.
If the packaging is not a box but, for instance, a stand-up pouch, a pillow bag, a bottle, etc., I usually have a 3D model in my extensive database. If not, I can always model one. In such cases, it's helpful to have a physical model of the packaging as a reference. Sometimes, 2D CAD drawings are available or can be obtained to assist in creating the wireframe model.