Given the complexity of driving steering servos, pid control filter setups and all the other tweaks required with the potential cost of an ooops moment... buy a marine autopilot that has a warranty / come back!
Raymarine do a good system which can track a gps route from a standard nmea interface. Most gps systems (not all) have a breadcrumb trail to return you to the start.

As a skipper, I can confirm that they do a very good job, providing you programme the gps correctly!