Know your clubface and path

Where is your clubface at impact? What is the path of the club? If your clubface is open to your path (path to the left, clubface aiming to the right for example), this is a main cause of slicing.

Drill: Slow down the swing (75% slower), and learn/feel to match the clubface with the path. The next time you practice, work on getting the path more to the right. Again, keep the swing very slow to feel the change.