Launch Ableton and open the project containing the tracks you want to sidechain together. Click the "Live Devices" icon, then open the "Audio Effects" section. Drag and drop the "Compressor" effect onto the audio track that you want to modulate with the sidechain.
Click the down arrow icon next to the compressor's power button to open the side chain section. Click the "Sidechain" button to enable sidechaining.
Click the drop-down menu under "Audio From." Select the source sound from the menu. If the instrument in the source channel contains more than one sound -- for example, if you're sidechaining to a drum rack -- select the source sound from the second drop-down menu in this section.
Drag the "Threshold" slider downwards to adjust the degree of sidechaining. The lower the "Threshold" control, the more Ableton modulates the destination sound.
Click and drag the "Ratio" control to determine how much compression Ableton applies to the sound. The higher the ratio, the tighter the dynamic range of the sound. Adjust the "Attack" and "Release" knobs to change the compressor's speed.
Click and drag the "Output" slider to set the volume of the sidechained sound. Play the destination sound to hear the sidechain in action.