First of all, there are different points in the city where you can take the ferry the most common is in Puerto Juarez.
The easier way is pre-booking a private transportation. There are many different companies that provide this service, they take you to your exact destination. Another option is to take a taxi, and the last one is going by ADO bus. Once you get to Cancun Town you have to take the bus or a taxi and to Ultramar Ferry.