Decide the main themes of the story: the who, what, when, where and how of the story. You do not need to have every detail outlined before you start writing, this can allow you to be more imaginative. Or you could map out every detail or encounter if this suits your tastes. Write all of this information down somewhere so you can refer back to it while writing your passage.
Start at any point of the story to write your passage. Some people like to start writing at the very beginning and work their way through a story chronologically, while others find this a particularly daunting task and would rather jump in at different point in the story. Choose a place to start that feels right to you.
Start writing on a word processing program or on a piece of paper. Do not worry at first about grammar, spelling and sentence structure, just start writing. Allow yourself the freedom to purely write without judgment and let your thoughts flow. If you get stuck, read over what you have written and image the possibilities for where the story could go. You could even write all of the possibilities down on a piece of paper as a brainstorming map.
Read over your passage and edit it for spelling and grammatical mistakes. Make sure the passage makes sense chronologically and logically.