Perhaps the most oft-quoted phrase from the Bhagavad Gita is the phrase “nishkaam karma” which means “the performing of actions without desiring the fruits of those actions.” In short, act, but without desire.
The importance of this idea cannot be over-stated. I understand the first shloka of the Isha Upanishad to mean exactly this. Let go of all desire, and rejoice, and live. God is everything. You have nothing else to do. One does not need anything more than this shloka to attain liberation. But one must take the shloka literally, at face value.
However, some preparatory work is needed to prepare oneself to understand this idea and actually put it into practice. I see friends who are deeply devoted to the Gita who do not have this foundation. For them, this phrase just remains an instruction, which is good to know but ridiculously hard, even confusing, to live by. Their natural question will be: why act at all if not for the fruits of the action?
I have understood that two things need to be contemplated upon first, before the idea of nishkaam karma is assimilated. First, one needs to understand na tvam deho, na te deho. This can be understood by a student of vedanta by applying well-tested processes like drig drishya viveka and avastha-traya. Second, one needs to understand Ma, accept Ma as she truly is. The moment one accepts Ma, one understands that there is absolutely no connection between action and its outcome at a human level — all outcomes are the result of the aggregate of all actions in the universe till then. The universe is a single machine.
Once these two are understood, then one realises that all actions done by “us” are done by our mind-body machines, all of which are parts of Ma. Ma alone acts. You and I do not do karma, we are not independent actors. Once this becomes clear, then the nishkaam nature of all karma automatically follows.