The first thing to know about slicing is if you slice-it is because the face of your club is open to the swing path. The most common swing path with slicers is when they swing outside to in, meaning for a right handed golfer- their club is swing too far to the left as they pass through the ball. Their balls start to the left and then slice to the right. If either their path or face angle are only moderately off this can be a very playable shot. Pull fade. The worse situation is when the player has an inside to out swing path, meaning the club swings too far to the right through the ball for a right handed player, and the face is open. This is referred to as a push slice, pretty much unplayable. The common denominator with both of these situations is an open club face. Often this a result of extending or cupping the left wrist on the back swing or unhinging to soon on the down swing and not being able to rotate the hands through the ball.
To help with closing the hands hit little shots trying to get the club to point in one of 3 ways, too far right, too far left and straight. After a little practice you will be better able to square the club with your full swings.