The most common slice I see is the classic “over the top” where the club cuts across the target line. The face angle is open to that path; otherwise you’d see a pull. Although this path is often caused by “throwing” your hands from the top (casting), I believe the root of the problem starts in the takeaway.
If a golfer picks up their hands in order to reach the top of the backswing, they tend to find themselves in a position where the hands are high and the only place to go is out and down. This causes an across the target line path that is disaster. Instead, try this: turn your left shoulder over your right knee (left-handed golfers turn your right shoulder over your left knee).
Starting the club back with a shoulder turn puts the club on the proper swing path/plane which in essence turns the club around your body. The swing path is an arc around your spine – turning your shoulders turns the club around you. I have found that getting the club in a good position at the top makes golfers less likely to cut across the target line. Instead, the club comes back to the ball on the proper path (from the inside), allowing for a better chance to square the club.
All that’s left is to release the clubhead through the ball, squaring it at impact. Let this happen, don’t force it…but that’s a completely different lesson. Good Golfing!