Do a little research if you want to make this fanfiction as close to the real characters and world as possible. Read the books or watch the movies. There are also many websites that offer good information about the books in detail. It might be easier to find information this way; try a quick search on Harry Potter Wiki (see Resources).
Open up a word processor. The type of files that most fanfiction websites, like FanFiction.net, take are .doc and .docx files. These generally work the best, although you can use a .txt in most cases. Remember that not all italicized, bold and underlined words will transfer when you post online.
Include a disclaimer. These are very important and must be posted in every chapter of every fanfiction that you write! It's fine to be creative about what you write, but make sure that the disclaimer states that you do not own Harry Potter and that you do not make any money from writing the fanfiction.
Basic information should be added at the top. Many writers like to include some information about their fanfic in each chapter. It's a good way to remind the readers about your fanfic before your chapter begins. Here are some items to include: fanfic title, chapter number and/or name, rating, brief summary of story, warnings and author's notes.
Begin writing. The writing process is different for everyone. Find your own voice, style and rhythm as you write. You can begin writing and develop a plot as you go, or you can have it all planned out.
Reread and edit your work after it is complete. Catch mistakes as soon as possible. Readers are forgiving, but they rarely tolerate writing that is not edited. Consider getting a beta reader (editor) for your fanfics if you are not comfortable or confident in your editing abilities. Many people love doing this as a hobby. With FanFiction.net, you can use the new Beta feature to find a good match.
Decide on how long your story will be. You can have your fanfiction be as long or as short as you want. Many fanfictions have multiple chapters that can be a novel by themselves.
Post your fanfiction for readers to see. You can get great feedback by doing this.
Continue to write and update your fanfiction until it is complete. There is no time limit on how soon your chapters should be up, but it is best to establish a pattern that your readers can get used to.