In the video you can see Nia Shanks demonstrating the proper sumo deadlift form. As she points out you want to actually set up with your shins touching the bar. You want to pull the bar in a straight line. You don’t want to try to squat the weight. You have to get into a proper deadlift position. You have to make sure that you hands are directly below your shoulders. They should be inside of your knees. You want to be sure that your grip is in the right spot, as you don’t want it to cause any issues when it comes to pushing your hips through at the top of the movement to lock out the weight
As she mentions do not set up too far away from the bar. This will decrease your leverage and the amount of weight that you can lift. You also have to be sure not to let you back round at all during the movement. You want your back to remain flat throughout the movement. At the same time it is okay to let your shoulder round forward. Trying to pull your shoulders back only increases the range of motion and decreases the efficiency of the movement. To lock a sumo deadlift out you really have to focus on flexing your glutes. If you find that you are missing weights at the top of any deadlift, chances are you have weak glutes that need to be addressed.