Karma is not something complicated or philosophical. Karma means watching your body, watching your mouth, and watching your mind. Trying to keep these three doors as pure as possible is the practice of karma.  Lama Thubten Yeshe, “The Bliss of Inner Fire”

That is the what to do.  This is the why:  ”so now also Christ shall be magnified in my body (Phil. 1:20); “the tongue is a fire” (James 3:6); “be transformed by the renewing of your mind” (Romans 12:2).