There should be a little rubber dust cap. It has been some time since I have done this, but if I recall correctly, you take it off and inside is a T shaped handle that you pull out and twist clockwise to increase pressure.
I would try to avoid any flaring if at all possible. It is my understanding that when the trans flares it rapidly wears your "clutches".
Someone very recently had posted how they changed the rubber connector between the plastic vacuum tube from the engine area going to the vacuum modulator valve. They said it didn't look cracked but was hard. They replaced it anyway and the results were "very impressive". There are MANY threads on this topic ... here is one:
Adjusting the trans mod valve