You need to lie down in semi supine on a firm surface; a carpeted floor or a rug will do, but not a bed as that would be too soft. It needs to be a firm surface to give your body adequate support and feedback about where it is in space.
The position is lying on your back with several paperback books underneath your head, hands on the belly and the knees raised. The feet should be about shoulder width apart and quite close to the body. Enough books under the head so that your head is level or tilted slightly forward rather than backwards. Use books or a firm pad, again you are looking for firm support. The books need to be under the bony bit of the skull rather than the neck.
Once in position, think and allow the muscles of the neck to release so that all the weight of the head is taken by the books. You want to release all the weight of the body and allow it to be fully supported by the floor. This will enable the back to open out, lengthening and widening, without you making any effort. This release in tension usually eases backpain and allows the body to regain its natural alignment, and improves posture. Do not try and push the back or shoulders down but just allow them to release onto the floor.
You need to lie down for about 15 to 20 minutes, time for the body to release onto the floor. Ideally you should do this twice a day. If you cannot manage the full time it would still be worthwhile as even a few minutes of lying down would help your back.