When you get there, ask others which entrance is the ferry to Gulangyu, because there are also tourist ferries nearby. Remember, you don't have to buy a ticket after you go in. You only need to buy a ticket in Gulangyu when you come back from Gulangyu. Don't listen to those soliciting, those are tourist boats, more than 100 yuan. Gulangyu Island is opposite the ferry, and it will be on the island in less than ten minutes.
In addition, I recommend you to take the BRT, because there is no traffic jam, and experience it by the way.
Have a nice trip! I am from Xiamen. I am used to taking a bus. I don't remember many roads. I can only tell you this. I believe it's easy to find. BRT has ambiguous platforms and service personnel. You can ask.