If someone asks you to do something for him, and he says "please do this for me, no more no less". That means he expects you to do exactly as he asked. He expects nothing more and nothing less than what he asked.
如果某人要求你為他做某D事, 他會說"請為我做此事, no more no less"。 那他的意思是要你正確地遵守他確切的指示做。 你不需要做多D或少D