To influence the tissues around the ankle whilst actively mobilising.
Have the person assume a half kneeling or split stance.
Using an interface like facial tissue, apply pressure in each of many directions to gain a feel for which direction feels less mobile. The skin and superficial connective tissue should have some stretch over the deeper tissues. Once you've found the main direction that has a restriction, apply a manual pressure in that direction and have the person actively lunge into dorsiflexion.
As always, assess and reassess after each treatment to gauge effectiveness of the chosen direction.