I am arriving by train at shenzhen north next sunday at 14.00.

You can take the metro line 4 to Citizen Center Station and transfer metro line 2 to Shekou Port directly. This trip will take around 50 minutes at a cost of CNY6/person. Don’t worry, it is quite easy to find the port when you arrive.
You can book the ferry ticket on the official website of Turbojet. The ferry from Shekou Port to Macau Outer Port will depart at 08:15, 08:45, 09:45, 10:30, 11:30, 13:30, 14:30, 15:30, 16:30, 17:30, 18:00, 18:30 and 19:30, and the ferry departing at 18:00 only works on weekends and legal holidays.