Corpse/Death/Savasana pose as it is known throughout the yoga domain, is not as easy as it might seem, particularly to the ones not aware of and/or not paying heed to it due to its very simple position.
You have to loosen/relax the muscles of your body completely to rest. You have to engage your mind to do this as well. Start concentrating your mind from the toe of your feet in a way that relaxation starts from your toe, to feet, to ankles, lower part to upper part of your legs to abdomen to chest, neck face and lastly the head. You just do this for several times till you feel totally relaxed and relieved and feel drowsy/sleepy. You might even sleep for a short while and wake up again leaving in a state of maze, peace and tranquility. This may go on for 5 – 10 minutes.
Suppose you are practicing any particular yoga practice e.g. bow pose, in a row of four times, you have to practice this four times in smaller spans after each time of bow pose and after four times of bow pose is completed altogether you take a longer span of Corpse/savasana practice. Generally four times of any practice is followed. But this may vary according to your physical and mental condition or temperament.
Corpse/death/savasana pose can also be practiced lying down on your front side as well.
Sometimes over-practice of backends can cause pain. Death pose is also to be followed after backend exercises!