Back extension

Get your heels under the bar and bring your hips and upper body as far forward as possible (your hips should be over the front cushion). 

Relax the body downwards, fold your arms across your chest and slowly lift the body up; ask someone to tell you when you make a 'straight line' if you're not sure, and don't hyper-extend by arching the back. 

Don't try to look up to the ceiling as this could strain your neck - avoid the Cobra position and keep looking down at the floor.  Lift up slowly, and go back to the starting position slowly as well to make the extensors work.  This works the muscles quite hard but feels incredibly relaxing at the same time