The most common type of slice out there is a pull slice. It’s an open clubface coupled with an out to in (over-the-top) swing path. If your banana ball is embarrassingly huge; starting to the left of the fairway and ending up in the right rough, take this drill to the range soon rather than later.
On the first movements of your downswing, take close notice to what your lead shoulder is doing. I’ll bet you it’s moving out the way quickly to the left. This will push your trailing shoulder forward, throwing your arms outside your target line, as shown in the image above in blue. This leads to your out to in swing path and ultimately your slice.
To fix this, we need to get your arms and hands moving in proper synchronization with the rest of your downswing motion. From the top, there should be a slight shift of your weight towards the target first, followed by your arms dropping down below your trailing shoulder before your shoulders begin rotating. This will ensure they are in the proper position and give you the best chance of hitting the ball along your target line. Your dropping arms will force your lead shoulder to rise, in turn causing your trailing shoulder to drop, followed by the natural follow-through motion to complete the swing. This is shown in yellow on the image below.
Take a look at your lead shoulder and see if it’s affecting your swing path. Give this drill a try, and let me know if it helps your pull slice guys.