Take off your shoes and loosen any tight clothing. Lie flat on the floor with your feet slightly apart and your arms by your sides, palms up. Cover yourself with a cozy blanket, if you like.
Become conscious of your breath. As you inhale, scrunch up a set of muscles as tightly as you can for about ten seconds, starting with your feet. Then, while you exhale, consciously let those muscles go—just let them flop. You're going to work your way up your body like this: legs, buttocks, stomach, chest, hands, arms, shoulders, neck, and face (including your tongue), clenching or stretching and then releasing. Allow all the tension to just melt away . . . until your entire body feels heavy, relaxed, and supported by the floor. You might even drift off to sleep, but do try to stay awake until you've worked your way through your entire body.
Once you become familiar with how relaxed muscles feel, you'll be able to release them throughout the day whenever you become aware that you're tensing up.
Beauty bonus: There are dozens of small muscles in the face, and it's one of the places we hold tension without realizing it. Notice if your face is tensed up at night, even as your head hits the pillow. Only when you consciously relax those muscles around your eyes, mouth, and jaw will you become aware of how you're contributing to the formation of wrinkles.