Golfers must understand that the swing that produces a slice is the result of poor fundamentals before the club starts in motion. The grip controls the club face and is usually one of the root causes of a slice, so gripping the club properly is paramount. From there alignment and ball position take center stage.
Slicers usually align to the left of the target with the ball positioned to far forward. Use alignment sticks to help get set-up correctly and make sure the shoulder alignment matches the feet. The ball should be inside the left heal for right handed golfers using a driver and more centerish when the ball is on the ground. The outside in path the club takes to produce a slice is usually deeply engrained so a golfer must start with half swings to help them eliminate the outside in path. Half swings with half speed all the while focusing on a more inside approach to the ball. If contact is improving go to three quarters then full swings.
Seek out your PGA Professional to better understand what you need to work on to eliminate the slice and play better golf.