How to work for liberating ALL living beings?
As per the "Heart Sutra in 8000 Lines", you liberate all sentient beings by attaining direct realization that in fact there are no any such "sentient beings" to talk about. "Sentient being" is only a nominal designation. Two keywords to study are anatta (sanskt., anatman) and shunyata.

Now, as a practice of paramitas (of generosity, forbearance, renunciation, loving-kindness, and equanimity), the commitment to staying in samsara and deferring one's own liberation until all sentient beings attain nirvana, requires complete surrendering of one's antipathy to samsara and craving for peace, complete surrendering of one's ego - which (the surrendering) paradoxically becomes the gate to Realization.