There are variations of the rear-delt fly, but if you're trying it for the first time, it's a good idea to start with a seated version to reduce possible strain on your low back. Sit on a bench or a chair with your knees bent, feet planted hip-width apart on the floor. Hold a dumbbell in each hand, resting them on your knees. Engage your core and tip forward from the hips, and as you do so, release the dumbbells from your knees, allowing your arms to hang straight down from your shoulders, with your palms facing inward. You'll hold this forward-leaning position throughout the exercise, so make sure your back is straight, forming a straight line from your hips to your head.
From this position, roll your shoulders back to keep them from slouching forward, then with a slight bend in your elbows, lift the dumbbells up and out to the sides, squeezing your shoulder blades together as you raise your arms. At the top of the movement you'll form a "T" shape with your arms and body, as if you had wings. Hold for a count, then carefully lower the dumbbells back to their starting position.