IF YOU HAPPENED to be in New Concord, Ohio, on Thursday, or in the more likely event that you watched a national newscast that evening, you would have seen Sen. John Glenn announce his candidacy for president. Mr. Glenn spoke in this pleasant midwestern town where he grew up from the John Glenn High School, located on Friendship Drive. If you inferred from this picture that Mr. Glenn continues to believe in the religious and moral values he learned in New Concord, and if you were reminded that he was the first American to orbit the Earth, you were getting just the message that the senator's campaign strategists wanted to deliver.

For the candidate's official announcement has become an art form. In the old days, a candidate would stride into the old Senate Caucus Room or the National Press Club and deliver a 10-minute statement averring his candidacy. Now he may do that, but he also will appear at a place and in a setting that is supposed to tell you something about his background and his issues. Consider how Mr. Glenn's five competitors for the Democratic nomination announced:

* Sen. Alan Cranston announced in the old Senate Caucus Room and at the Cardinal Cushing Student Center at St. Anselm's College in Goffstown, N.H., and talked about ending the arms race. That tells you that Mr. Cranston is hoping to finish high in the nation's first primary in New Hampshire, a contest in which most of the Democratic voters are Catholic, and that his major issue is arms control.

* Sen. Gary Hart announced from Denver and beamed his speech over C-Span and the Cable News Network, thus stressing his links with technological innovation and the western state he represents.

* Former Vice President Walter Mondale chose to announce in the chamber of the Minnesota House of Representatives. As he declared, in a statement all the TV networks would use, "I am ready to be president of the United States," it looked as if he were. He was speaking on a rostrum high above a legislative chamber with the speaker and senate president sitting even higher behind him. Mark him down as the candidate of experience.

* Reubin Askew announced at the National Press Club and at the territorial governor's mansion in Tallahassee, where he served eight years as the governor of the nation's seventh largest state. He then flew to Miami, where he raised much of the money that enabled him to rank third among the candidates in fund-raising this year despite his lack of national name identification.

* Sen. Ernest Hollings made his announcement at the Midlands Technical College in Columbia, S.C., an institution established when he was governor more than 20 years ago. Thus Mr. Hollings underscored his interest in job training, his record of practical accomplishment and his emphasis on how "Reagan is putting America out of business."

In effect, each candidate has prepared a little television program, intended to provide you with some favorable message. You can be as cynical about these mini-messages as you like. But they do indicate what the candidates would like us to think of them, and at this stage that's at least as important to the voter as speculation about which county chairman is backing whom in Iowa, or who gave the most rousing speech at the latest cattle show.