The old Masters often used an underpainting to lock in shapes and values. One method is called a wipeout where you draw contours and and shadow shapes onto the canvas, then apply a thin layer of raw umber or burnt umber. Using cloth from an old t-shirt you wipe out the lightest values first and then the other light and mid-tones. You can add back in darks that got wiped out using a brush. Below is a quick study I did after a portrait by Rubens.