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Before you begin tensing your muscles, it may be helpful to engage in deep breathing. Breathe all the way down into your abdomen slowly, focusing on the sensation of your breath. Keep taking deep breaths until you feel a sense of calm and are ready to begin. Go through the list of muscle groups in sequence. As you work through each muscle group, tense the muscles for about five seconds each, then let go and let the muscles relax for about 10 seconds.  Start by making a fist with your right hand and tensing your forearm. Lift your right forearm to tense your upper arm, as though you were flexing your muscles. Repeat these steps for the left hand/arm. Tense your forehead by lifting your eyebrows as high as they can go. Squeeze your eyes closed tightly. Open your mouth as wide as you can to flex your facial muscles. Lift your shoulders up towards your ears. Then push them back, trying to touch your shoulder blades together. Tense your stomach muscles. Then move to your hips and buttocks, squeezing your muscles together. Tighten the muscles in your right thigh, then (slowly) stretch your calf muscle while curling your toes at a downward angle. Repeat for your left leg/foot. When you've finished tensing and relaxing your muscle groups, take a few moments to keep taking deep breaths. You may also want to stretch out your muscles to resume normal blood flow.  Open your eyes and regain a sense of your surroundings. Don't stand up too quickly, or you may get disoriented or experience tense muscles. Simply sit for a few moments, breathing and stretching.

Summary:
Slow down your breathing. Tense and hold your muscles in sequence. Keep breathing and open your eyes.