"Professional manner" means behaving and conducting oneself in a respectful, knowledgeable, and appropriate way, according to the standards and expectations of a particular profession or workplace. Full definition
When working as a volunteer, you must conduct yourself in professional manner. (cvtips.com)
I ran my business in a very professional manner while providing a relaxed environment for my clients. (livecareer.com)
Good interpersonal and communication skills and acts in an independent and professional manner with the ability to influence others including senior management. (targetjobs.co.uk)