First of all, you'll need to find the player you want to follow. You can easily find another player in-game by clicking the magnifying glass in the bottom right corner.
Type their name into the search bar and hit 'search'. Clicking on their name from this list will bring up the information box about their boat on the left side of the screen.
In the boat's information box, look in the very bottom corner. You should see a small avatar icon with a (+) symbol.
Click the button and it will turn green along with a (?). This is asking you to confirm you want to follow that player.
Clicking it again will add the player to your list of followers and the button will turn white marked with a (-).