Savasana or Corpse Pose is said to be the most important pose of the practice. I agree, which is why you'll normally find a longer Savasana in my classes and me teaching Yoga Nidra any chance I can. Why?
In this fast-paced world we live in, it's so important that we take time to slow down and do nothing. We've been trained to do, be and have more, faster, better and not to settle until we have it all. While this is not necessarily negative (though it can be if left unchecked), it does need balance. This is why a pose that can seem like the easiest, can be the most challenging (lay down on the floor... how hard can that be?)
Like any other pose, I can give you many, many details of how to set it up. Ultimately, it's yours to feel into and here are some suggestions to play with and see what works...
Lay down on the floor and test out whether flopping your arms and legs open is enough to relax neck, shoulders, low back and hips. (Even though it's called Corpse Pose, you don't need to squeeze onto your mat as if it's a coffin). Really let your legs and arms sprawl open and let go of all that tension that gets picked up from other activities and experiences. Especially if you have the tendency to make yourself small or take up as little space as possible, let your body spread out. If more adjustment is needed, try letting your chin go heavy or sliding the back of your head away from your feet more to lengthen and relax the back of your neck or a small block, blanket or pillow under your head. Flatten your shoulder blades on your back like you're sliding them down and out, so your shoulders go heavy and release the weight they may be carrying. Slide or tuck your tail towards your feet to let your low back relax. If that's not enough, put something under your knees or bend them and have feet flat on the floor, so your low back can go heavier towards the floor. Having something under the knees also lets the femur (thigh) bones go heavy and sit back into the hip sockets where they better relax the nervous system rather than being forward and ready to be active. It also can help if your knees tend to hyper-extend or get uncomfortable. A blanket over you (or just feet or hips) can help to keep you comfortably warm and act as extra weight to help you feel grounded and relaxed. Similarly, an eye pillow can help relax the eyes and face by shutting out light and grounding or adding extra weight. If none of this works, you can find another position (like curling up and rolling over on your side, laying on your belly or anything else that suits you to relax).
Once you're set up to relax and be as comfortable as possible, then the practice of Savasana can be a little easier. If you're not fully comfortable, you'll want to get out or squirm around and not be able to gain the full benefit of this practice. When the body relaxes, it helps the mind to relax (and vice versa because they are not separate).
Savasana is a time of integration of all the other poses that came before as well as every event that came before. This means that we get to more fully receive the benefit, wisdom or lesson from any other experience. It also means resting from any effort or strain and releasing tension or anything else that's no longer needed. The nervous system can be balanced, blood pressure can be regulated, digestion can be aided, fatigue can be removed and so much more. The trick is to stay awake and aware of the body relaxing without doing anything (meaning not letting the mind wander away to plan all kinds of other tasks or remember who said what, why, when...). Of course, the mind is very good at being busy, so the simple instruction is to stay or keep coming back. Most importantly, be gentle with yourself.
When you practice the art of Savasana and letting go, you make room to receive. Some would say it's practicing dying or an ending, so a new beginning can emerge.
May you release, restore, receive and re-member (come back to yourself) in Savasana,