The basic "rule of thumb" in tennis has traditionally been-
respond to topspin with topspin, respond to slice with slice and this is
what I would recommend if your shots are going long as you describe.
I'm sure you have seen this happen with the pros, once one of the players
slices, every ball thereafter is sliced back back and forth, until one takes the initiative
to hit a more aggressive and "chancy" shot.
If you do want to try imparting topspin to low slice, then keep your racket face approximately
perpendicular to the court surface. Do not open the racket face by tilting it "up".
If the ball stays really low you will need to "whip" through it and try for a placement
deep in the opposing court.
If you are having trouble with this, as you say, just reading about it is not enough
to turn a weak shot into an aggressive response. You need to practice.
Have a friend stand at the net with a bucket of balls and hit deep "slice volleys"
alternating between your forehand and backhand. Try driving through them with topspin.
Once you get the feel of that, then move up closer to the net and try doing the same.