Position feet under bar with very wide stance. Squat down and grasp bar between legs with shoulder width mixed grip. Face forward while positioning shoulders upward with arms straight, chest high, hips low, and back straight.
Pull bar up by driving feet outward while pulling chest up. Extend knees when bar passes knees. At top of lift, when torso is upright, drive shoulders back and chest up. Return weight to floor by bending hips back and knees pointed outward, while keeping chest high and back straight. Repeat.
Use shoulder width grip since narrower than shoulder width grip will make it more difficult to lockout at top. Throughout lift, keep hips low, shoulders high, arms and back straight, and knees pointed out same direction as feet. Also, keep bar close to body to improve mechanical leverage. With wide stance, push feet out to sides and not down to prevent knees from buckling inward. Pushing knees out will allow hips to travel toward bar more quickly while improving leverage.
During rep training for powerlifting, do not merely tap weight to ground, but instead, let barbell settle to ground each rep to sufficiently train range of motion through initial slack of heavy barbell. See similar exercises: Deadlift and Kettlebell Sumo Deadlift. Also see Sumo Deadlift vs Conventional Deadlift and Deadlift Strength Standards.