Staying in the moment or the present is a transcendent instance in golf.
I want to say it's a transcendent action but that may imply that it's a physical action and really it's not.
It's an aligning of your mind to the very actions that you are engaged in - at that moment - without any thought which strays to either the future or the past.
If you smashed your thumb with a hammer, you may well be so absorbed with the pain that you have no room for any other thought, thus you are in the present, all be it an unpleasant one.
As soon as you begin to think of consequences - "How can I play golf tomorrow with this smashed thumb!" - you are no longer in the present but have strayed into the future.
This is an example where you are moved into the present automatically by an event, but intentionally moving your mind into the calm and focussed present, needed for golfing excellence, is not easy.
So now you know that staying in the present is a mental state which must be moved into and sustained, and that being fully present in the moment, will lead to fully focussed playing in the "Flow" or "Zone" state where peak performance is achieved.
The problem is that for the vast majority of golfers getting into and then staying in the present for even the short duration of one shot, is a serious challenge.
Either we are worrying about that last bad shot, and will we hit another one, or we are nervous (terrified?) of the consequences of hitting a bad shot.
Like watching the ball fly out of bounds in our minds eye as we dread the possibility of that swing and the consequences of that result.
At the same time as we are digging this hole for ourselves, we may be pulling the dirt in on top of our own heads by analysing poor swings mechanically and trying to put that right on every swing.
Paralysing.
Clearly the flow state, playing in the zone, is paralysed and along with it any chance of playing anywhere near what we are really capable of with unconscious performance.
If you could play golf with as much confidence and as little conscious thought, analysis and worry as you do when you tie your shoe laces, you would play some very good golf.
Oh how we worry about the past events and imagined futures! The irony is that the only thing you ever have is RIGHT NOW, here in the present.
Neither the past nor the future really exist, they are both just mental constructs made by our minds in order to "position" ourselves in relation to the present time or moment.
Imagine this.
If you really did not care about the outcome of any golf shot, ever again, if you paid no mind at all to the consequences of hitting any shot, good or bad.
If you just focussed on the process of steps (pre shot routine) that lead to your mind swinging your body and propelling the ball to your target, how do you think you would play? By drilling a solid, repeatable pre shot routine that you use for every single shot in both practice and play, you are developing a major tool that helps lead you into a flow state, in the present moment for the 30 seconds or less that each shot requires.
Some of the quickest and most effective ways to enter a state of flow, or the Zone if you prefer to call it that, is to use "triggering" and "anchoring" techniqes from the field of NLP.
In addition, develop your own focus ability by choosing focus developing techniques that appeal to you.
There are many available and a search of the internet should reveal a whole raft of choices.
If you are really stuck I would be happy to give you some suggestions.