Insert a compressor on the track you want to duck
Route the sidechain source track to the compressor track
Open the compressor and select the sidechain input
Enable sidechain input from the source track in the mixer
Lower the threshold until the compression triggers
Set a fast attack
Set the release to match the rhythm
Adjust ratio to control how much the volume ducks
Fine-tune knee, if available
Use the mixer routing knob to control sidechain send level
Check the gain reduction meter and adjust as needed