Idiomatic use is as a metaphor and informal. “Smooth sailing on my new job” or “She had smooth sailing on her new job” are both awkward constructions. I’d suggest
My (or her) new job was smooth sailing.
When asked “How’s the new job going?” the cool response is a simple
perhaps with a hand gesture for emphasis where the speaker starts with the elbow bent, hand next to the shoulder, palm flat and downward, and extending the hand out and on a gradually downward curve ending at about elbow height. Imagine the speaker showing an airplane coming in for a soft, gentle landing. For fullest effect, make a relaxed, easy going face with one or more of head tilted slightly back and to the side, eyes narrowed, and lips pursed slightly to indicate “The situation is completely under control.”
Another answer suggests plain sailing as an alternative. In the U.S., I have never heard plain sailing used. I would guess that it is used in the same way in the U.K. but am not qualified to state as much. Idiomatic usage is a world all its own.