Through service and sacrifice.
We see that in the lives of Christ, Buddha, Krishna ... and lesser mortals: Gandhi, Mother Theresa, Martin Luther and the so called non-spiritual: Darwin, Gorbachev, Mandela ...
Their way were service and love. Greatness we gave to them, they did not seek it.
Others have an opposite kind of greatness that we call something else and instills fear: Like Hitler, Mussolini and others that I am sure you know.