Click on and launch Cubase. Click "Project" and then "Channel." Select the "Group Channel" option from the drop-down menu. Click the arrow in the "Group Channel" box, and select the "More" option. Click on the "Quadro" option from the drop-down list, and click the "OK" button.
Press the "F4" button to bring up the "VST Connections" window. Click the "Groups/Effects" tab at the top of the window. Right-click on the the "Group 1" channel, and select the "Stereo Bus" option from the drop-down menu. Right-click on the group again to select the "Stereo L/R Bus." Close out the "VST Connections" window.
Click on the mixer icon on the right-hand side to display the mixer. Click the "Group Channel" and right-click to open the "Insert Effects" option. Click the "Sidechain Compressor" option from the drop-down menu.
Set the "Threshold," "Ratio," "Attack" and "Release" to your preferred settings. Select the instrument(s) in the song that you want to sidechain. For example, select the bass or bass drum to ensure those sounds are heard above all other sounds.
Right-click on the the instrument, and select "Sound Effects" from the menu. Select the "Stereo L/R Bus" from the list. Press the "Play" button on the "Transport" bar to listen to the track. Click on "File" and "Save" on the top menu to save the track. All sidechain settings are saved with the track.