Mind power building is simply about using what is yours. There are ways you can build "reserves of power" but a more direct way to build power is simply to recover what is already yours.
You realize that this often means taking it back from someone else who has been "carrying" it for you.
Getting your power back means taking responsibility. It means gaining back control over your time frame, personal space, thoughts, emotions, actions and behaviors.
As you can see, basic forms of power have nothing to do with controlling someone else's life. On the opposite.
Disinvest from what is not yours and can't influence, and focus on gaining back control over what is yours.
The real magic of this is that: having power over what is yours is one of your most basic human right.
Sometimes you give away that power because you don't want to hurt anyone or you don't dare to use it.