The method should be precise so that the same story is given to each employee at the same time. First, the company should call a meeting with all department heads where senior management breaks the news. Managers should be educated on how to tell their teams. They need to be prepared, know the party line and get it right. Make it clear that the talking points given in this meeting are the only words you're comfortable with managers using to communicate the news.
Role play with managers and prepare for any possible questions.
People will have questions, and there may be confidential information that cannot be disclosed. But whenever possible, management needs to be forthright so as not to incite panic or speculation. Once the word is out to managers, it should be communicated to the rest of the company quickly, ideally that same day. The more time that passes, and the more people who know, the more likely it will leak. Every person who knows is a liability when you're trying to keep a secret.