It's a technique I learned from Wasim Khan (https://creaturerigs.com/).
Harry Houghton had also made a video tutorial outlining the process (https://vimeo.com/94663329)
Copy the "rr_deformableControls" folder to your scripts directory.
You can check your scripts directory by typing "internalVar -usd;" in the command line.
Execute in ScriptEditor (in a Python tab):
from rr_deformableControls import core
c = core.deformingControls()
You can highlight those lines and middle click and drag it to your shelf to create a shelf button.
Once that's done, all you need to do input your mesh (the deforming mesh) in the "mesh" field using the "<<" button.
Then, you can specify your control. Finally, you need to make a selection of the faces you want to associate to that control by making a selection of faces on the deforming mesh and populating the "faces" field.
When you're done, click setup and it should setup the deforming control.
* The 'Include mesh transformation' checkbox can be used if you have transformation values on your geometry or it's parents.
This requires the "matrixNodes" plugin to be loaded in the plugin manager. The plugin is packaged with Maya.
* The 'Retain original shapes' option can be toggled on or off if you'd like to keep the current control curves, turn it off to replace all the shapes in the control.