Three options for strengthening your slice backhand
If you want to hit a topspin backhand two-handed, first learn a weak-handed topspin forehand. Then just pull it in so you can keep your dominant hand on the racquet.
If you want to learn an one-handed topspin backhand, be advised that it's a very difficult shot unless you use an extreme grip. (Martina Navratilova was #2 in the world before she learned that shot. Rod Laver "learned to control it" in his last years as an amateur -- when he was winning grand-slam titles.) And people with good slices don't use an extreme grip.
So if you want to strengthen your one-handed backhand, either learn to topspin it via a semi-western grip (all four knuckles near the top of the grip at contact), or simply learn to hit a flatter slice. A slice that rolls backwards very slowly can be a very strong shot indeed -- especially nowadays when so few players are used to receiving them.