We should serve the guru by following his instructions.
Srila Prabhupada gives the example that if the spiritual master asks for a glass of water and we think, “Let me bring him a glass of milk,” then we aren’t serving properly. A disciple may think he is serving by bringing the milk, because milk is better than water, but if we actually want to render service to the spiritual master, we will simply do as he asks. That’s the definition of service. By rendering service in such a way, we come closer to understanding the guru’s heart and thus closer to the spiritual master.
(Lecture SB 4.2.21, Moscow 10.27.1996)