Application Decommission/Retirement Process

Hi everyone, I’m searching for a process or checklist that will tell me what I need to do or who do I need to inform if an application is to be retired. I must put together a project plan. I’m at a loss. If anyone could send me an example of a project plan or a list of things that must be considered when retiring an application.
Background info: the aplication will be replaced by a new application. The application is internal facing only and it is used by help desk personnel to assist callers with application/business process questions. The outgoing application consist of data in the form of articles. It is a database

I appreciate your assistance.