I tape a copy of the rough pencils to the lightbox and then place the bristol board over it.
Because I can see the image underneath I draw over it onto the board. I'm also able to make final changes to the pages at this stage. I usually go slower with this stage to make sure that I'm doing it correctly.
Once the drawing is on the board and I have finished with the lightbox, I add small details - like strands of hair- and the blacks.
This final stage can take some time but is usually the quickest part of the process.