A dozen new mayors got together with a dozen old mayors yesterday at the opening of a four-day Institute of Politics conference aimed at teaching how to make the transition from candidate to chief executive.

"Once you become mayor, it's hard to study how to become one," explained Lee Robinson, mayor of Macon, Ga.

The eight biennial Seminar on Transition and Leadership for Newly Elected Mayors, cosponsored by the IOP and the U.S. Conference of Mayors, is trying to provide City Hall rookies with this kind of knowledge that can only come from someone who has been there. The program includes discussions of everything from ethics and dealing with reporters to running a police department and dealing with natural disaster.

In Robinson's case, he attended the conference two years ago when he was first elected, and returned yesterday to offer advice from his own experience.

One of the best lessons Robinson learned, he said, came from the 1987 mayors' conference when Mayor Jerry Abramson of Louisville, Ky., told him about "Mayor's night in, mayor's night out." This technique involves scheduling times for citizens to come to City Hall and for the mayor to go out into the community on a regular basis.

Newly Elected

When Abramson was a newly elected mayor and attending the conference, he said the best advice he got was that the most important member of his staff--and the person he had to select most carefully--was his appointment scheduler, because this would be the person deciding how he handled his time.

Other participants in the conference are Charleston, S.C., Mayor Joseph Riley, who is scheduled to speak today about how to deal with a disaster like Hurricane Hugo, and Boston Mayor Raymond L. Flynn, who will speak tomorrow on drugs and crime.

Robinson said of the seminar, "The timing's excellent because it allowed me, at a time when I was very receptive, to receive timely and pertinent information about how to be a mayor."

History Makers

The conference attracted a few mayors who have made political history, including Norm Rice of Seattle and John Daniels of New Haven, both the first Black mayors of their respective cities, and Karen Vialle of Tacoma, Wash., and Martha Wood of Winston-Salem, N.C., both the first female mayors of their cities.