The slice is probably the most common swing fault among high-handicappers and it's important to understand this particular ball flight.
It is a shot that curves left to right for a right-handed player and right to left for the left-handed players, and it's generally caused by sidespin produced by a combination of an outside-in swing and an open clubface at impact.For this discussion, let's talk about right-handers.
Many players that slice the ball will try to aim left by opening their stance to keep the ball in the fairway. While this will occasionally keep your ball in play, this actually makes the problem worse because the open stance further promotes an outside-in swing path. Even if you square the club in this alignment, you will probably pull the ball straight left.
Instead, align your feet so they are parallel (square) to the target line or close your stance slightly by pulling your right foot back a bit. This will help to promote more of an inside-out swing path and reduce the sidespin you're putting on the golf ball. And always remember, practice makes perfect.