With all of the trending glass bobs, getting sleek, straight hair can be a challenge. Panel cutting towards the ends of the hair is a great technique to remove bulk and collapse the ends of the hair to avoid a bell shape.
Avoiding the part line and the front hairline, pick up a thin section of hair above the ear. Holding the section at 90 degrees, cut about an inch of hair from the length. Cutting at 90 degrees elevation will remove weight without leaving a line of demarcation. You can repeat this process, moving up the head shape as desired.