A slice is a weak shot that flies high and curves from left-to-right. Golfers slice the ball because they swing too upright/steep into the ball and their arms reverse rotate, which opens the clubface and imparts cut spin on the ball. Unfortunately, many golfers also make their slice bigger by cutting across the ball in an attempt to try and compensate for the open face.
To fix your slice, you need to figure out if you take the club inside on the backswing, lift it straight up, and come down steep into the ball or if you swing too steep all the way through, up and down?
If you are steep all the way through, you need to round out both your backswing and downswing. Practice swinging the club more around your body on an arc, rather than straight back and through.
If you take the club inside-up-and-over, you need to try to reverse the loop. Instead of taking the club back flat and down steep, you want to take the club back a little steeper and down flatter.
Once you feel comfortable making practice swings with your new motion, try to hit shots that draw from right-to-left. A draw is a more powerful shot and by exaggerating the opposite, you will keep your swing in the middle. I'd also recommend starting with a 5 iron and teeing the ball up when you are trying to level out your steep approach into the ball.
*If you are still having trouble, take a slightly strong grip, one where the "V"s formed by your thumbs and forefingers point more toward your right shoulder. Make sure your grip pressure isn't too tight because the tighter you hold the club, the less active your hands will be and the more you will tend to block the release of your shots. Keep your back to the target at the top for an extra second as you start your arms and hands down toward the ball and consciously rotate your hands through impact.
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