This drill is for improving the slice forehand groundstroke technique, placement, and consistency.
The cones I use in this drill are to be taken as targets but do not focus on knocking them; that is not your priority.
I will practice hitting 5 balls cross-court, 5 down-the-line, and 10 alternating side to side.
Technical elements to follow:

- continental/hammer grip,
- 90-degree between the forearm and racquet,
- prepare with the racquet head above your shoulder so you can swing from high to low toward the ball to impart slice/under-spin, 
- it helps if you hold on to the racquet with both hands (non-dominant on the racquet neck) until just before you are about to swing down and forward to meet the ball,
- consider it like a forehand volley to which you add a longer backswing and follow-through.

The slice forehand groundstroke is not an easy stroke because it requires good control of the wrist but, with practice, you will feel the ball and how tight you want to hold the racquet at contact.

Secret: The longer your backswing, the tougher will be to control the wrist at contact, therefore I suggest you keep the non-dominant hand on the racket neck until you are about to swing toward the ball. As you gain more confidence, you can add a longer backswing and let go with the non-dominant hand earlier, but keep it short in the beginning.