There are two layers: the business case and the project plan.

They're two separate things. Don't create a project plan without the business case, because you'll struggle to get buy in. Asana: The beginner’s guide to writing an effective business case You can ask for a ready-to-use template if the company has it for the business case or create a new one. In there you have to answer the questions: what is the objective, why do we do it, what is the value. One of the most important skills you can learn is how to tell the story of your initiative in a way that people with very limited time can understand: what's in it for me, what do I need to do and when, and what criteria and requirements are needed.

After presenting, take notes of the questions you get and use them to improve your plan. For the project plan, start with the end in mind and work backwards. What are the things that need to happen to achieve the end? A Gantt chart can be useful to map the milestones and understand the capacity you need. How much time you need, who will be involved, the key people and at what stages to inform certain people.