If you are new to drawing you propably have some dificulties with drawing what you see. You rather draw the idealized image that you have in mind ‒ let´s say you are to draw an eye of a person. If you are new to drawing you would draw the image of a stereotype eye that you have in mind rather than the eye that´s in front of you. This is a mistake a lot of newbies make because they are not used to switch between the left brain and the right brain.
The left brain side is the one where verbal and analytic processes happen, the right side of the brain is the place of visual and perceptual thinking. Betty Edwards has made reseach on this topic and with her instructions everyone is able to draw much better after just a few exercises. These exercises could be flipping the reference upside down so that you take the context (i.e. an eye) out of it. You then just draw what you see.
Drawing what you see and not what you think it should look like is the basic element of realistic drawing – if it’s a Bargue Drawing, a Cast Drawing, a figure drawing or a portrait. You need to learn how to see distances, angles, lengths and shades. You can either stupidly measure and copy without thinking but that is not your end goal. Your end goal should be having the ability how to see and not how to measure. Measuring is learnt quickly and easily – learning to see is a much tougher process but the more rewarding one.
Take a look at this website, which is dedicated to the principles of Betty Edwards teaching. Especially look at the Before-After page, where the students had to make a self portrait before and after the exercises. You can see a huge difference after just about a few exercises.