DRAWING THE CAGE

 

Using the any of the Grease Pencil tools draw a shape that will enclose either the entire object, or only the parts that will be deformed.

draw cage3
Instead of a single stroke, you can also draw using multiple strokes. Press "Shift+K" and turn on "Join Strokes" to create a single Cage object.
multi1 multi2
Instead, if you keep the "Join_Strokes" off, you can also create multiple shapes.
multB1 multiB2
You can decrease the geometry detail using the "Simplification" slider.
simp2 simp3