1) For classic bob. first do the middle parting and take a A-line section at the nape. Note that the A shape should be pretty border than usual. Just short of a horizontal section.
2) comb out this section properly and cut at 0 degree at the desired length. this will be your guideline.
3) while deciding the length of the guideline, it should be noted that, once the hair is dry, hair might lift up. so depending on the hair type cut your guideline approximately half inch to one inch longer than you actually wanted.
4) now only for the hair at the back of the head, keep taking sections parallel to the A-line section you had taken and cut these sections at 30 degree.( you will include some sections from your already cut previous section so that it serves as a guideline). while cutting these sections you will stand at the back of the client and take the sections in the center of the head, exactly 30 degree above the place where you took your guideline.
5)Now move to the side of the client. for cutting hair form the side you will keep on taking horizontal sections. and go cutting from bottom to the top. ( you can divide clients hair from the side into two parts. and then take horizontal sections first in one part and then move to another part.)
6) Take first horizontal section from the bottom. obviously you will have some hair form the previously cut section from the back as the guideline. cut this section straight at 30 degree keeping the length in sync with the length of the hair taken from the backside as guideline.
7)keep taking horizontal sections as you go up and cut at 30 degree keeping the length consistent.
8) Repeat the same procedure on the other side.
9) Now take the hair from the corners, where clients back and the sides join. merge this hair with the hair form the back and sides.
10) your haircut is done. Now lets do the checking.
11) Take hair form one side of the head and check if there are any stray strands of hair coming out. cut off any stray hair or tails coming out. Repeat the same for the back and the sides.
12) You will observe that since the hair is cut at 30 degree. the hair automatically gets slightly in-turn at the ends giving a graduated look.