When you entrust your boat to a marina, their trained staff know how to winterize your boat. Be sure that you put safety first, when attempting to winterize the motor(s) yourself. Carbon Monoxide is a silent killer.
You cannot see or smell the gas. In most cases there are no warning side effects to alert you to dangerous consequenses--unconsciousness followed by death!
We would recommend a digital readout carbon monoxide monitor in the boat with you, during and after running the motor(s). It will show you when the CO levels have reduced to a safe level, so you can proceed. If not, you will have ample time to ventilate and/or get out or stay out of the CO fumes, before it is too late.