Using scissors on yourself at an amateur level is surely extremely risky but some guidelines may help you get closer to perfection!
1.The scissors must be used at an angle. This will help you grab the perfect layer of hair to snip as well as give you a sharp edge. It is convenient and also the smarter way.
2.The back of your head is truly an impossible task so take help for the back no matter how skilled you get. You cannot see the back of your head and so this is not the option for you by logic.
3.Start with the sides and once you are done compare both to ensure the length is same. Both sides should match for symmetry of the face. Also by starting at the sides you can verify the level of skill you possess. If you feel its not working out, you can repair the damages with a professional. If the sides are not working out, you can alter the top to look more interesting and convert the sides into a fade or undercut using a trimmer.