To make a cobweb we first make straight lines emerging from a central point. These lines are then interconnected at almost regular intervals by curvy lines (curving towards the center) running perpendicular to the straight lines.
After having made the cobweb, sketch over it while applying enough pressure to produce an outline on the backside of the sheet. Its not necessary that the outlines on the backside will appear so don't keep sketching over and over. The idea is to deepen the sketch outline.
Now rub it off. Even after rubbing off you can see the outlines of the sketch. Now you can apply shades and lights as you wish. The cobweb will a