The Akai usually comes with an editor software (available on the product page) where you can configure the MIDI parameters of the pads, knobs, etc.
Depending on the VSTi you are using, you'll basically have to map or match the MIDI notes of the AKAI pads to the MIDI notes of the VSTi plugin.
As simple example:
- Akai Pad #1 sends MIDI note C1
- The Kick in the plugin is triggered by MIDI note C1
Some plugins might also provide a learn function for assigning MIDI controls (pads, knobs, etc.) of the AKAI direclty.
Or you can have a look at the Learn function in Reaper's plugin window (see links below).
In addition, some videos which should be helpful in general:
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https://www.youtube.com/watch?v=gVPVY7LYAMs
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https://www.youtube.com/watch?v=AkCNbRNa1nU
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https://www.youtube.com/watch?v=F0E6qieRaPQ