Do you have any striking experience at allŅ If not, go to a gym and learn basic footwork and striking first.
The key for the backward walking punch to inflict damage is that the forward momentum of your opponent has to be bigger than your backward momentum. Then your punch must be very precise right on the button and your opponent shouldn't be prepared to get hit.
So training wise you have to pay really much attention on your timing, the synchronization of your movement to your opponent's movement and the precision of your punches. You can try to train it with your trainer holding the mitt walking towards you while you do evasive movements and out of these hitting the mitt.
But don't expect to knock people out like Ali or Silva. There is a reason why they are considered the greatest of their time.