How do I shave my own head?

So, first time shaving my own head, I have a no.2 on the top and no.1 around the sides and it usually costs me £10 a time - so I bought a £10 shaver and I have done it myself.

My issue is the back - without being able to see it, I can't shave it properly myself so I am going to have to get my flatmate to do it... just wanted to know if anyone found a good way of getting the back done evenly by your own?