[Spiral]

Having taken some lessons on drawing from life last year I have now begun to practice in the way I was taught. I set up a jar, a baseball and a small box and I try to draw them. I am trying this in order to grasp the concept of tones better. And although with quick inaccurate scetches I might do well I have a really basic problem when dealing with a challenge for a more accurate drawing, I never get to add any tones because I am stuck with the measurments, I am constantly being puzzled with the procedure.

So, is there any tutorial or any advice you can give me on the subject?

Thanks.