You can combine the structural sketch with the finished line work on the same sheet of drawing paper; however, this may involve erasing, which can alter or damage the surface of the drawing paper.
To avoid unnecessary erasing on the drawing paper, you can work up the structural sketch on a sheet of sketch paper and then trace or transfer the image onto the drawing paper for the final line work. As an added bonus, you can use a copier to adjust the size of the structural sketch before tracing or transferring for the final drawing.
To use a lightbox to trace the structural sketch onto the drawing paper, tape a sheet of drawing paper over the structural sketch and place it on the lightbox. Its light will allow the image to show through, which you can then sketch onto the drawing paper, minus any unwanted lines.
To transfer a sketch, tape the structural sketch to the drawing paper, then insert a sheet of transfer paper (also called graphite paper) graphite-side down between the structural sketch and drawing paper. Use a 2H pencil to go over the necessary lines of the structural sketch to transfer the image onto the drawing paper.
To make your own transfer paper, evenly cover one side of the tracing paper with lines from the 2B pencil. To bond the graphite of the pencil to the paper, smear a cotton ball, slightly damp with rubbing alcohol, over the surface of the tracing paper.