Stand straight holding the dumbbells in front of your thighs, with palms facing them.
Bend your elbows until the weight is up to shoulder height.
Slowly lower back to just before full extension and repeat.
Using momentum from the body.
Fully locking out elbows.
Elbows moving around too much.
Stand straight with your feet shoulder width apart, holding the dumbbells in front of your quads with your palms facing them.
Start by bending your elbow and bring the weight up until your hands are approximately level with the shoulders. Then slowly extend the elbow back down to just before the starting position to keep constant tension. Your hands should remain roughly in line with your shoulders on the way up and down. Keep your wrists aligned with your forearms and your elbows should remain fixed to your sides throughout the movement. Repeat for the desired amount of repetitions.
Avoid using excessive momentum from the body or bending the wrists.