This delicious posture can be done without a mat on any hard surface, and you don't really need too too much space. All you're going to need is a towel, blanket, or sheet, and your beautiful self.
First, take whichever "prop" you have, fold it, and then roll it into a cylindrical shape that is approximately 3 inches thick and at least a few feet long. You want it to be AT LEAST a few inches wider than your shoulder blades on each side for the pose to be held comfortably.
Position yourself in front of the rolled up prop with your knees bent and soles of the feet on the floor.
Now, find the very bottom of your sternum (breast bone) and place a finger there. Draw a straight line from that point around your rib cage until you're touching your spine. If you move your finger from side to side you will graze the very bottom of your shoulder blades, your "wing tips". This is the part of your back that you want to be resting on the cylindrical roll up you just made.
Lean that part of your back onto the roll up, keeping your knees bent and soles of the feet on the floor, allowing your shoulders to pour over the roll up onto the floor. Tops of the shoulder blades should meet the floor. If they don't you should adjust the thickness of your roll up.
You should feel a "sensation" in the space in between your shoulder blades, and it may be uncomfortable (BUT NOT PAINFUL) at first. You are placing a great amount of pressure on some very deep, very thick upper back muscles where we all hold A LOT of tension. If you don't feel any sensation whatsoever, you should increase the thickness of your roll. If the top of your shoulders do not touch the floor, decrease the thickness of your roll.
When your settled at the right thickness, bring your arms out to "T" shape with the palms facing up, then bend to a 90 degree angle. (This is called "Cactus Arms")
breathe, smoothly in and out through your nose.
relax, your forehead, your eyes, your jaw, your throat.
surrender, allowing your body to melt into the floor and the support, no tension, no muscular effort at all.
This pose feels soooo good and is a stimulating, invigorating, deep, but passive backbend. A great way to experience the benefits of deep backbending/heart opening without the risks that come along with them, especially if you're a beginner. It can be practiced in the morning in lieu of a cup of coffee, or any time you need a jolt of energy! Also great to relieve muscular tension and pain in the upper back.
You're also opening your heart center, or chakra, which is said to govern the heart and circulatory system, thymus gland and respiratory system. In terms of the subtle body the heart chakra represents the seat of your higher emotions like true and unconditional love. You may take your attention to your sternum area and mediate on the following mantra if you'd like: "Yam", pronounced Y-ah-m. NOT like that orange potato we eat during the holidays in the US. :P
If your mind wanders, that's ok, bring your attention back to the mantra or to the the inhale and exhale.
Hold for at least 3 minutes.
To come out of the pose: Gently, gently, GENTLY, roll to your right side. PAUSE, take a few breaths here. Then, letting your head be the last part of your body to come up, gently press yourself up to a sitting position. Find a comfortable cross legged seat (you can sit up on your prop if you'd like), close your eyes and breathe. You may feel a jolt of energy after stimulated those DEEP upper (thoracic) back muscles.