It sounds so simple, huh? It can be, but for some it is SO HARD. My favorite thing to do is get down on the floor, prop my legs up on a table/bed/couch/ottoman and just chill out.
Focus on doing your really amazing 360 breathing and moving those ribs laterally (out to the side). Hang out as long as you feel comfortable, but I usually end up spending anywhere from five minutes to an hour (if I fall asleep!). So deeply relaxing.
The back can relax fully, too, which is great for tension/pain. Let your legs really flop to the sides and pay attention to *anywhere* you are holding tension. I store mine in my left groin and jaw 😎 So mindfulness is key here. Put something under your head if it is uncomfortable for your neck or your ribs thrust too much.
See if you can feel your breathing down into your pelvic floor. This will help relax things, big time. To me, it feels like my bum is expanding/opening, and that is how most people describe the sensation in my experience. This is a good thing for all my tight pelvic floors! You have to learn how to let go and allow it to expand fully in order for it to effectively contract completely and at the right time.
Runner, lifter of children, PTA, CPT, PCES, pelvic health zealot