This important posture, often called "corpse pose," is typically the last in a yoga class. It's many benefits include lowering the heart rate and blood pressure; relieving head ache, anxiety, or fatigue; allowing the entire body to relax; and calming the mind.
To lay in savasana bring your body down to the mat or rug or wherever you are practicing and onto your back. Allow the shoulder blades to move down the back and the neck to the elongate. Let your feet relax outward and your hands to rest at your sides palms up. Gently close your eyes and unclench your jaw.
Comfort is critical. If laying on the floor is not comfortable for you, use blankets, pillows, or bolsters to prop the body. A thin pillow under your head may help ease any pressure in the neck. A bolster or rolled blanket under your knees may ease low back or hip discomfort. Covering up with a blanket will keep you warm. Use an eye pillow to help your facial muscles and eyes to soften.
Allow your breath to relax. Sink into the mat as you let go of any tension in your body and your mind. For the duration of your stay in savasana, your task is to simply let go. Nothing left to do. Nowhere else to be. Namaste.