It depends on the person you are. I am the type of person who sits down to write and the piece writes itself. This may not be the case for you or anyone else. As others have said, just, start writing. Write a sentence, then when you're done with that sentence write another sentence until you get a paragraph.
It doesn't matter what you write, what part of the bigger story it is apart of, to start, all you have to do is take the leap of faith and actually begin writing, otherwise, you'll never write.