A bob is a blunt haircut with straight edges, so the hair needs to be clean and uniformly wet in order to achieve a proper cut. Wash and condition the hair, and towel dry.
Divide the hair into seven sections. One section is the top. Create sections for each side, for the left and right parts of the crown, and for the left and right parts of the nape. Leave out a 1/4-inch section of hair all the way around to create a guideline cut.
With the head positioned forward, comb the hair at the nape over your fingers. Without pulling, cut the nape hair to the desired length. Follow this length all the way around for the guideline, with the head up for the sides. Hair should be all the same length.
With the head positioned forward again, take down 1/4-inch sections of hair from the nape and then crown sections of hair. Comb them over your fingers with the guideline, and cut.
Once the back is cut, take down 1/4-inch segments from the side sections of hair. Comb each over your fingers with the guideline, and cut. Take down the top section. Part it down the middle, and comb the hair evenly on either side. Cut this hair to match the sides.