I trace the sketch onto a full size (18'" by 24") sheet of vellum or Arches onion skin and then retrace the final lines onto a gummed block of Arches 40 Lb rough 100% cottan watercolor block, using a graphite transfer sheet (which works like carbon paper, but leaves a light grey indelible line).
I use a ballpoint pen to transfer the traced immage so I can see which lines I have overtraced. This process is damnded tedious with absolutely nothing to recommend it, requiring brute tenacity and patience rather than skill.