A simple grid can make life so much easier when studying from a photo reference. When drawing from life you will need to practice other methods of measuring if you are after an accurate drawing. Those methods could also be used when drawing from a reference image but using a grid speeds the job up. After drawing a grid on the support to the same ratio as one on the photo reference you are ready to go. Next draw an out line drawing by continually checking accuracy against the grid. Mark out all major internal lines before blocking in the main shadow shapes.