Canada is a party to the Hague Service Convention, in force since 1989, and it is one of the easiest countries in which to serve U.S. process. Each province and territory has its own Central Authority, and Canada has not objected to the alternative methods in Article 10, so documents can also be served directly: by a process server in the English-speaking provinces, and by a huissier de justice (bailiff) in Quebec.
Because direct service is permitted, most U.S. matters in Canada are served personally, in a way that looks familiar to U.S. counsel, with an affidavit of service prepared to your court's requirements. The formal Central Authority route remains available and returns an official certificate. The main thing to watch is Quebec, which follows civil law and has its own language requirements.