Decide on what it is that you want to write about and why you want to write it. This will help you to concentrate on a real life event to change into fiction.
Decide on the purpose for the scene or story that you want to write based on a real life event. You could be using this real life event to create the climax of a story, the introduction of a character, the introduction of a plot point, or any other major part of a story.
Choose a single real life event that is relevant to the purpose of the piece of writing that you are aiming to complete.
Break this real life event down into its key points. These key points should include answers to the who, what, when, where, why, and how questions about the event and the motivations of the people involved in the event. If the real life event that you have chosen is a natural event like an earthquake or tornado, list the scientific causes of the event in the place of the main figures' motivations. List all of these key points.
Decide on what key points and main figures you want to include in the piece of fiction that you are aiming to write. Choose those things that are the most relevant to your writing purpose. Put these key points in a list.
Embellish the key points that you have selected, so that they perfectly suit the purpose of your story. If you have chosen an event that involves a main figure whose motivation was related to money and you are writing a story about the destructive power of greed, make that main figure deeply greedy. If the event is a conference that was attended by a few heads of state, and you are writing about an earth-changing event at an international summit, then change it so that it was attended by all of the most important government figures in the world. If you are writing about something more mundane like a trip to a grocery store and you want to show that your character is facing feelings of unfulfilled desire, then put him in situations where he is confronted with unaccessible options that appeal to him (his favorite brand of ice cream is on sale but he has only enough money for bread, for example) --- even if this is based on real life and there was no such sale. Do not overembellish your key points, since this could lead to the creation of caricatures of people and events.
Make a list of all of the embellishments that you plan to make relevant to your key points. You should now have three lists: one of the key points of the real life event on which you are basing your writing, one of the key points that you are going to use in your writing, and one of the embellished key points.
Take your list of embellished key points and write an outline of the piece that you are aiming to create. This outline should include how the real life event that you have chosen is related to your story and why this real life event is important to it, in addition to the key plot points of your story.
Write a draft of your story based on your outline. Edit your multiple drafts until you are satisfied with the resulting piece of writing.