If the first and only location isn't doing well or needs fixing, then don't get into a second location.

If your first location is doing well, then that's something that comes from business planning. There's no magic formula. You look carefully at the market in general, the market at the second location, what it would cost you in expenses and assets and related development costs, how much your fixed costs would increase, and how much your sales would increase. Make carefully researched educated guesses.

Tim