For the most part it is a good idea to have a plan. For most anything I create I have a plan. I plan my paintings, my drawings, my jewelry, my shop improvement projects, my home projects like landscaping, and painting… well you get idea. PLAN FIRST.
Visualization is key to the planning process. One needs to see or feel the end in order to have a successful start. My advice is to visualize the end the go forward with a plan and be confident. You may need to alter the plan as you go, but that is alright.
The plan is the real life manifestation of your visualization and as always life and technical ability get in the way of every plan. That is where the confidence comes in. Be confident that you will succeed even if you have to start your painting over or your life over.