Steps to tracking a straightline with a simple heading controller?

So i've got a simple heading controller for a boat in Simulink. Input reference heading and then it gives the heading response.

So now i'm trying to get the boat to track a straight line and I'm a bit confused on how to actually implement this into Simulink.

So ideally what I want is to define an arbitrary x,y starting position for the boat, and a straight line. Then need to compute the position error and somehow convert this into a heading which is then fed into the heading controller, and have the position reevaluated each time step. I'm just not entirely sure how to get this going and would appreciate any guidance.

Many thanks