"He who fails to plan is planning to fail"
Sir Winston Churchill
There are many quotes that are constantly changed/misinterpreted. This is one of them. Usually it gets paraphrased to "Failing to plan is planning to fail." Either way the statement is correct. A plan is a necessity for a leader. As well as a "plan B" for when the inevitable happens and something goes wrong. In Project Management (and the military) there is a Risk Assesment Matrix that is also part of the plan. The plan should be detailed, should be in chronological order, and should be made available to everyone who will be impacted by it. At the very least the plan is an exercises to identify the tasks necessary to complete the project, and in what order they need to occur.
The plan can be a writeup in a notebook, a checklist in Outlook, a detailed project plan in Primavera, or anything in between. The only thing I can tell you for certain is that if you don't plan you will miss something and then have to scramble to fix it (that isn't to say that you won't miss something; but with a plan you will miss less).
Of course, no plan is perfect. Things will happen and the leader needs to know how to adapt. A quote to talk about later will be "No plan lasts past first contact."