While the other methods mentioned work great and are even more practical for specific edits, I figured I'd chime in and make you aware of a very simple method for automating pretty much any knob in FL, any knob on a VST, or practically any function capable of changing/moving whatsoever, all with the same one action.
In this example, I'll use the tempo. All you have to do is:
1.) Click on the tempo and move the value up or down, then return it to where it was. This will be your "last action," so don't do anything else afterwards.
2.) Click on "Tools" at the top of FL Studio.
3.) Go to "Last tweaked."
4.) Click on "Create automation clip."
All done. :)
You can use this method to automate any last thing you "moved." It could be the cutoff on a VST in FL, it could be the volume, it could be turning an effect on or off - pretty much anything. Once in a while, something might not work, but that is very rare in my experience.