Slicers and people that pull shots, tend to come over the top on the downswing. The results is that the club swings from the outside to in path. If the clubface is open on this path, the ball will fly left to right. If the clubface is closed on this path, the will ball go straight to the left. You will want to learn how to sequence your swing so that you learn to swing the club on the correct plane, so the path is more inside to out. This drill will help you learn to swing the club on the on the correct path.
Stop Coming Over the Top Drill-
Set up in your golf stance holding a club just with your back hand (right hand for right-handed golfers).
Extend your front arm out in front of you.
Swing the club back to the top of your swing just with your back arm.
Initiate your downswing with your lower body so that the back arm swings under the front arm. Note how the shoulders are still square to the target line, not open, where the right arm has come over the top.
Do this slowly at first, then pick up the pace when you feel comfortable with the move.
Repeat for 8 to 10 repetitions.