Correct. Leave engine in neutral. Your keys should be on a welded ring. Take keys from ignition and switch on prop alternator key. Green light should show charging. To stop turn off and extract key. Green light may still show charging. Stop by putting gears into reverse. Into neutral and start engine.
Don’t know what size your alternator is. Mine is a 50amp. It only puts a couple of amps in until I’m over 5 knots then it starts producing. Crossed the Atlantic with batteries at 100% all the way - all on the prop alternator.
Ocean Hobo SN96