Does not matter which transportation you take, you have to come to Split ferry port and take tickets for Supetar. After driving 50 minutes you will reach Supetar. Than continue driving the car until Bobovisca on the sea. It will take you 15 minutes. There you will meet hosts and they will drive you with small boat to the Kupinova bay.
If you are without a car the hosts will meet you in Supetar and transported you to the final destination.
Have a nice trip.