The simplest volley drill is for either two players or four players to stand at the no-volley line and volley the ball back and forth. Each player should attempt to hit the ball to the other player in a manner that will allow them to keep the ball going. At all levels, the goal should be to keep quite a few balls going between misses.
For beginning players, this might mean you are hitting the ball fairly slow and high and possibly even to the forehand. As you improve, you might hit the ball a little firmer and even try to hit to their backhand more often. You will find that all players at all levels will do best if you don’t hit the ball right at them.
As players improve, you can hit the ball harder at each other and intentionally hit some to the backhand and some to the forehand and some right at the other player. If you are having long rallies, you can get more aggressive. If your opponent is starting to miss too much, then you should slow the ball down until he/she is successful again.
With only two players, you should practice not only volleying the ball straight ahead, but also crosscourt using both backhands and forehands. With 4 players you will get to practice both, but you should practice both from the leftside and the rightside of the court.
Remember, the goal is to practice and keep the ball going, not to hit so hard the other player can’t get it back! However, you should be noticing any particular weakness they have in case you ever play them in a tournament.