As we approach the waist of the guitar, the sides would again bump up in width and as you approach the upper bout, they are cut down once again.
Side Variation is Less for Classical Guitar Sides:
This undulation will be far less pronounced in the Classical Guitar construction as the back plate arch is less severe than that of the acoustic guitar. and the taper of the sides from tail block to head block is only about 1/4 of that of the acoustic. (Acoustics vary quite a bit with side taper and you can design your sides to best suit your own tastes).
With the combination of the side taper and the back arch, you really need to project the different points of side heights on a template.
Do Not Taper Your Sides In A Straight Line:
The most important thing to note that the tendency would be to draw a straight line from the height of the tail block to the low point of the head block. But, the problem with that is you will wind up cutting into side wood that needs to be retained to accommodate the back arch.
The Alternative Method:
Another method of trimming the sides to shape is have the sides run flat from the tail block to the widest point of the lower bout, and then taper from this point to the head block dimension. Then you take a sanding block and gently round the edges, fit the back and trace the contour on the sides and keep adjusting until everything fits perfectly. As you can imagine, this takes a bit of time.
In the next article we will explore how to project your points onto the template and making a side contour template.