Sumo deadlift

Stand with a very wide stance and grip the bar with your hands inside your knees, feet much wider than shoulder-width apart.
Drop your hips, keep your chest up, and maintain a flat back to prepare for the lift.
Drive through your heels to lift the bar, keeping it as close to your shins as possible.
Stand up completely straight and pull your shoulders back at the top of the movement.
Lower the bar by pushing your hips back and bending your knees, resisting the weight on the way down.

Tips

Before you pull, position your hips low enough that your shoulders are directly above or slightly in front of the bar, with your chest facing outward rather than toward the floor.
Keep the bar in contact with or very close to your legs throughout the entire lift, from the floor to lockout.
Point your toes outward to match the angle of your knees so that your knees track over your feet as you drive up.

Allowing the hips to rise faster than the shoulders at the start of the pull causes the torso to tip forward and the lift to resemble a conventional deadlift rather than a sumo pattern.
Letting the knees cave inward as the bar leaves the floor reduces the leg drive available for the lift.
Jerking the bar off the floor instead of applying steady upward pressure often causes the hips to shoot up abruptly, breaking the setup position before the bar has cleared the ground.