This, although also a difficult question is somewhat easier to answer in a slightly more direct way, begin by showing up. Once an idea of a story, be it personal or a short story has begun to shape itself in your head, the best way of honouring it is by sitting yourself down and capturing on paper all the thoughts around it that come to your mind. Section off an amount of time in your day to sit down, in a quiet space and work at developing that story, whatever details of it come to mind.
Some writers have said that what works for them is creating a mental picture of the story and then setting off to describe the image in their minds. For some writers it’s a line that plays itself over and over again in their head, which once they have put on paper, makes it possible for them to write even more lines. For others still it’s a sensation, a feeling, something they have seen or experienced and their desire to describe it.
What ever it is for you, that is the gift you have that will guide your beginning.
At this point it is important that you do not pay too much attention to whether the story is making much sense, but focus on capturing as much as possible of the initial details or fragments of your story. Only once you have captured these will you be ready to start shaping your story.
Some useful techniques include brainstorming your idea on your own, bouncing off ideas with a person or people whose ideas you trust and so on. But ultimately as with the ‘Where do I begin?’ question, only you can decide the how.
And the how is made manifest when you actually take the effort to do it.