I live in Montezuma, and I do this trip regularly.
I haven't read through the other postings in detail so I may repeat or contradict some info. But I do know this journey very well
Firstly - before I get into transport info - you don't mention dates, so know that Ylang Ylang is closed right now for the next 2 months. I think until the beginning of Nov. Their sister hotel is El Sano Banano in town, which remains open.
The best way is the direct bus from the Coca Cola station. It leaves at 6am or 2 pm (that is current, but the times do sometimes change). The sign on the bus may also say Malpais/Santa Theresa, but it is the right bus. The bus goes on the ferry, so you reboard the same bus on the other side of the ferry so you can check your luggage and not have to worry about it on the ferry. You may have to change bus in Cobano (7 km from Montezuma where the road splits to Malpais) - but the driver will indicate what to do.
The ferry company is called 'Tambor' but the ferry goes to Paquera. There is no ferry that actually goes to the town of Tambor. If you take the direct bus, it is easy and you don't even have to think about anything until you get to Cobano when you need to pay attention to directions from the driver.
I would highly recommend the early bus, just because it gets you to Montezuma before dark so you can get your bearings.
If you want info on other options, just let me know, I have done the trip that way too, but I assure you, the direct bus is the easiest way if you can manage to meet the times.
Hope this helps