Yoga term: Savasana
This pose is called corpse for a reason. A corpse has no life, no mind, no reason for tension. It is completely relaxed and surrendered. This is the state of mind to be achieved. You can lay however you like in order to be comfortable and focus on relaxing. There are alignments but relaxation is the most important aspect of this pose.
When lying down keep the hips and shoulders square. Tuck the shoulder blades so they are flat on the floor. Arms at your sides with palms down.