If you have an idea about what you’d like to do, then that’s great: you have an idea.  You don’t yet have a plan, however.  You don’t have a plan until you write it down and work it out.  When it’s in front of you, and you can analyze it, critique it, and revise it, then and only then do you have a plan.

Write it down.  Then you can put it into action and judge it based on its results.  Keeping it in your head keeps it away from your eyes and out of your course of action.  Write it down!