For Dumbbell Curls, choose two dumbbells of equal weight and put one in each hand. Start with your hands hanging down at your sides, palms facing inward. Lift both arms, rotating your arms so that your palms eventually face upward as you lift. Stop when your elbows are completely bent and then slowly bring your arms back down to resting. Then repeat. Alternatively, lift each arm separately, alternating which one is lifting.
Make this easier by using lighter weights.
Make this harder by using heavier weights.