I first learned about this technique from David Leadbetter (not in person but from one of his home videos). When it comes to a good swing, golfers should consciously think about two main things: face angle and club path and their relationship to one another; both of which you’ll probably hear me use frequently on this site.
Michael Breed from the Golf Channel demonstrates a good drill below to help you develop a good inside-to-out club path by “hitting through the gate.” Davide Leadbetter used balls but probably best to use tees just in case your F’up. haha
FYI, this isn’t a cure-all fix for your slice. Note that you can have a good inside-to-out path but if your club face is open when you strike the ball, you will probably still slice the ball. However, if you can develop a consistent path (something I still haven’t seemed to master) then you can shift focus on making sure your face angle is square at the bottom of your swing.
Good luck! Let me know if this helps! – WH