Working on your follow through won't do anything. Follow through work only changes how pretty your finish looks.
You need to work on whatever causes the follow through to be poor. If you're lifting up before impact, that can be a problem, for sure. Need to keep yourself down long enough to hit the ball properly, but it doesn't matter if you look up after impact during the follow through
Lots of tips here on slicing, but that main thing you need to do is lead with the lower body and follow with the shoulders arms and hands. I'm not the guy to explain all that though.. there are the 5 swing keys, and there are lots of threads explaining those. Also, posting a swing in the myswing thread will help you out a lot