NEVER sit on your block and lay back.
ALWAYS lay down first, then slide the block under you.
Supported Bridge ~ Salamba Setu Bandha Sarvangasana
Getting into the pose:
Sit on the floor with knees bent. If you have a belt put it on around your upper thighs. Tighten it to where your legs can still open to hip width apart. Lower yourself down to the floor and lay on your back with your knees bent. Lift your hips in the air and place a block under your sacrum. For Restorative I like to have the block on the lowest side but if you want to try it on the medium height or on the highest height that's ok too. Just make sure you are comfortable and nothing is feeling pinched in your low back or neck. Lengthen the back of your neck. Resist looking side to side. Rest your hands by your side.
Settling into the pose:
First, make sure that the block is under the sacrum (the big triangle shaped bone at the base of the spine), not the low back (lumbar spine). If you have a pinched feeling in the SI joints (low back or sacrum area) try lengthening your tailbone toward the backs of your knees so your body is in one long line from shoulders (on the ground) to knees (in the air). You also might need to lower your block height. Let the hands rest by your sides, close the eyes, and give your mind the job of watching the breath. Stay for a few minutes or as long as you are comfortable.
For a more dynamic pose:
If you want to add movement and/or more sensation to this posture here is a way to go a bit deeper:
walk your shoulders under you and clasp your hands together behind your back to open the chest. Stay for a few breaths and then slowly release.
Getting out of the pose:
To come out, lift the hips up, slide the block out from underneath you, drop the hips back down to the floor, rest here for a few breaths then take what ever movement would serve your body: hug the knees to the chest, take a twist, full-body stretch, etc. When you feel ready, roll to one side then gently push yourself up on one side back to an easy seated pose.