Here's the easiest method of side chaning a drum to another source (this is meant to be quick since a lot of videos and tutorials irritatingly long and spend 20 minutes without getting to the point).
Create a channel for your 'beat' source (i.e. kick drum) in the mixer. Select it. Right click on the TARGET field you want to link the kick to and select 'Sidechain to this track'.
Add the 'Fruity Limiter' mixer plugin to the TARGET (in the example above that would be 'strings'). At the bottom where you see LIMIT and COMP, choose COMP. In the 'Sidechain' field, select the sidechain we've just created. Set the loudness and envelope roughly to how you see it set in the below picture (adjust accordingly to preference):
No 20-minute video tutorial required.