Kids always take this differently. There is not really a best way to do it. As a whole the best way to do it is together with your spouse in a comfortable setting. Just be honest with them without too much detail and let them respond or possibly ask questions. The kids need be the lead in the conversation. It is ok for you to be nervous. Don't let it get the best of you. Keep things simple and centered around you and your spouse made this decision together.. even if you didn't.
Relax Eat Think Act normal React.. Smartly. Do something different. Emulate. Do Work.