I got a haircut and now it looks freaking squarish! I want to know how to cut the back of my hair?

Okay I went to get a haircut, I told him not to cut my sideburns, but he cut the hair aroiund my ear, if you get what I mean. Now my hair looks squarish at the sides (but with sideburns) and the back of my hair is really long, poking out. I don't want to go back there again and waste another 25 bucks so I want to do it myself. How do I shorten the hair at the back? I just want to cut it a little bit, and I'm scared I'll mess up, I don't want a curved hair at the back! Any tips? And do you think my hair will look better once it grows longer? o.O I mean, my friends actually called me Frankenstein but obviously he was exaggerating lol.