Sidechaining effect can be applied through the Mixer window of FL Studio. Different FX plugins can be used to add that effect. In this article, I will discuss how to do it using the Fruity Limiter and Gross Beat plugins.
The Fruity Limiter way...
First create a track with a lead instrument, using one of the sound-generating plugins. Switch to piano roll for that channel, and add a long continuous note.
Link that channel to one of the Mixer tracks.
Add the Fruity Limiter plugin to one of the FX slots for that track.
Select a track you would like to sidechain to, a kick drum track is a common thing to sidechain to. Then right-click on the Send button in the lead track, and choose Sidechain to this track in the pop-up menu.
Open the Fruity Limiter plugin parameters for the lead track.
Switch to Compressor settings, by clicking on the COMP button in the Envelope section of the parameters.
One of the parameters that will appear is the Sidechain input box. Right-click on that box. You will see all the tracks your lead instrument is sidechained to. Select one of those tracks. If you sidechained it to a kick drum, select the mixer track linked to the kick drum track.
Under the Loudness section of the Fruity Limiter parameters, find two knobs called Thres and Ratio. Turn the Thres knob almost to the extreme left. Turn the Ratio knob to the extreme right.
Hit the Space bar to preview your composition. You should now hear your lead instrument going up and down in volume along with the kick drum.
A picture is worth a thousand words, a video is worth... even more. So here's the video from the creators of FL Studio showing what I just wrote.
The Gross Beat way...
Gross Beat plugin allows for a much simpler way to add a sidechaining effect.
Just select a mixer track linked to a lead instrument you'd like to apply sidechaining to. Then add Gross Beat plugin to one of the FX slots for that track.
Select Sidechain from a list of available effects.