Draw the hand first, then draw the arm, then the head. Once you have the elements in place, go ahead and erase and correct any lines that are incorrect in your estimation. Break the drawing up into simple cylinders, so that in your minds eye you can see how the various parts fit together. Take it slowly and try and see why the fingers (and arm) appear to look the way they do. If you end up drawing one finger too fat, go ahead and erase and redraw it. Seeing the underlying structure of a figure takes time to master, so keep practicing.