Negative ground goes to a convenient solid bolt into the motor chassis. For me, on my '89 Mariner 25hp that was a starter motor attachment bolt above where the cables entered on the port side of the engine. I've also seen it on a similar engine going to one of the flywheel guard hold down bolts.
Positive goes to one of the large solenoid contact studs, opposite the contact stud going to the starter motor.
