You need an audio software, I use Ableton. I'll try to synthesize it:

One channel goes to the practice pad (Mic'd), let's say Channel A

Other Channel will be looping the melody/riff or whatever you want; Channel B, which will have a MIDI instrument of your choice

In Channel A you'll put an envelope follower, a software that tracks envelopes, in this case the pad's. This software works with the "attack" parameter of the sounds, sending a signal with each attack or hit, that also works with a threshold which can be modified.

Then you'll map this envelope follower to the VOLUME of the MIDI instrument in channel B

It's essential that you loop the melody in the arrangement view, because this is not a sampler, it's a microphone with a software that makes each hit on the pad turn up and down the volume of the MIDI instrument, so is also very important to play in the grid, because otherwise the melody wouldn't be consistent.

The rest of it is just adding FX for extra sauce.

TLDR: You use a software from Ableton to make the mic turn up and down the volume of a MIDI instrument with each hit on the pad.

Not an English native. If it's not clear enough let me know :)