Begin with the end of your task, goal, project, or dream in your mind by mentally envisioning it and creating a written description or picture of exactly what you want the end result to look like when the project or vision is completed.
The 7 Habits of Highly Effective People - Goodreads
To begin with the end in mind means to start with a clear understanding of your destination. It means to know where you're going so that you better understand where you are now and so that the steps you take are always in the right direction. "Begin with the end in mind" is based on the principle that all things are created twice. There's a mental or first creation, and a physical or second creation to all things. Take the construction of a home, for example. You create it in every detail before you ever hammer the first nail into place. You try to get a very clear sense of what kind of house you want.