If you are still working on your strength, do not reach up as high to begin with as you want to be able to pull up to the point your top arm is at eye/forhead level. Pull your lats straight down as hard as you can while pulling your rib cage into your body. You must be using core arnd back strength primarily but every muscle in your entire body should be engaged like a plank on the floor. If you body starts to tilt it means that one of your shoulders is disengaging so make sure to pull down evenly on both!