This is how it works:
One layer is a joined curve consisting of all the out most lines in the scale model. You want to fill/hatch this shape with a solid color and keep an outline thickness.
The other layer holds all the details of the model. You want to have this layer on top of the first layer.
The combined layers create the nice looking scale model. These two layers have to be over the layers containing your drawing.
PRO TIP: Give the lines in the layer with the outlines a higher thickness value than the inner detail lines. This make them look extra great! All models displayed on this site are done that way.
Since these models are only 2d, this is how you will acctually experience the models. The two former pictures were just to explain. Now you can group the model and move it around without having to cut lines from the drawing in the back!