To fully understand the pre-shot routine you need to know what it is for let's break down it's purpose in to 3 three segments.
1. PREPARATION These are the thoughts that take place while you are deciding what you want to do. What club do I choose? What is my target? What distance do I have to carry it? What ball flight am I expecting? Wind, terrain, stance, pin location, lie, and a myriad of other factors come into play while you are in the preparation phase of your swing thought. Before you move on to execution thoughts, you must be totally committed and totally confident that the choices you made during preparation are complete and are the right choices. You must believe that the the choices you made will result in a shot that you know you can do, and do it well.
2. EXECUTION This is the golfer's version of the pilot's pre-flight checklist. Take an overall census of your swing fundamentals; grip, stance, alignment and relax!! This is happening just before you take your stance over the ball. Finally remind your self to do those 2 or 3 things that you have been working on with your coach or on the practice tee and take one or two rehearsal swings to create the short term memory you need for confidence. Now you are ready to focus.
3. FOCUS This is where you create the confident state that generates the great swing. From a few feet behind the ball you begin your focus process. Focus on the target! Stare from the ball to the target, see the ball's intended line of flight, visualize the club's swing path and breath smooth and easily. Take your grip, lead hand first. Focus on the grip and get it exactly how you want it, based on the decisions you made earlier. Now add the trail hand precisely like you have practiced it. Calmly walk to the ball and place the club head behind the ball with the face of the club exactly on the target line, step into your stance with the trail foot first then the lead foot. Get into your well practiced and confident stance and mood. Focus on the ball! Pick the smallest spot on the ball that you can see and commit to strike the ball on that precise spot. Focus on the swing! Each golfer must find that one key thought that enables them to freely swing the club with confidence. It may be a tempo or rhythm thought. It might be all about swing path. Or perhaps you need to focus on the shoulder turn or the finish, the important thing is to hold that one thought vividly in your mind until the swing is completed.
The brain can only hold one thought at a time, and it will always have a thought. Either the thought you put there or something random and not helpful that just creeps in. Like "get over the water", "don't chunk it", or "where's the beverage cart?".
Practice those three steps, make them automatic. Until they are automatic create a cheat sheet, to remind you of the things you should be thinking about before every shot.