We finish a southern Italy tour in Naples and starting another Best of Adriatic tour the day after. What is the best way to get from Naples to Ljubjana?
You’ll be flying. A train is 12+ hours. Start looking at flights, they are not inexpensive.
Check Rome2Rio. The best solution seems to be to fly from Naples to Trieste and then to take the shuttle to Ljubjana.
Starting in Croatia and you ask about Ljubljana? Maybe explain in some detail please.
That is where the RS Adriatic tour starts. I assume the OP is taking that tour.
Best of Adriatic tour starts in Ljubljana.
What is OP?
I think the earlier comments refer to you posting your question in the Croatia forum when it relates to Ljubljana which is in Slovenia.