The start of a painting is probably the most important part of executing it. Everything that happens after fails or succeeds based upon the strength of the structure underneath.
All the thought and care that's put into the start will make the rest of the work a lot easier. More often than not I fall into the trap of thinking ahead to the finish before I consider all the work
It's impossible to overstate how important the start is. Not that changes aren't made as one goes along to the finish but there is a great deal of assessment made from the very start.
Below is a video of a painting that I started in Autodesk Sketchbook as well as a list of books about different artist and their approaches to painting.