Grab chunks of hair in the back, close your eyes, cross your toes (since your fingers are busy) and cut, hope for the best. I usually cut until it feels as short as I want it and then I grab a mirror and check to see how wonky it is. You can use the scissors and cut up and down to start evening it out.
Everything else that you can see... just hold your scissors upright and cut and cut until it looks just right.
I have been doing this for about a year now and it's been good and it's been bad. One day when I save up enough I'll get a real haircut. Until then, this diy cut will have to do.