Remove some of the straight lines that connect the circles so that the number of lines radiating from each circle equals the number in that circle. When you are done, none of the connecting lines can cross each other. For example, the 3 in the lower left hand corner has three lines radiating from it, so none can be removed. But the line from the 3 to the 6 forces us to remove the line that crosses it (from the 2 to the other 3).