This video explains what a Project Charter is and how it relates to the Project Lifecycle and Project Plan, and demonstrates how to write a project charter, starting with understanding the project before documenting the Project Charter (know the stakeholder and project boundaries/scope). Project Charter templates can be found here: https://templatearchive.com/project-charter/
I recommend producing a draft work breakdown structure to help you know what to document in the Project Charter and information on how to build a Work Breakdown Structure is here:
https://www.youtube.com/watch?v=0EGE1diWazo&t=28s